You asked:
"How do you know you have enough "

Enough is such a thin film that the plant appears slightly dusty. Gobs of the stuff is excessive.

Why are you using it on Hostas?

Be careful not to breathe that Sevin dust. It is dangerous if you inhale it (dangerous if you don't, also IMHO). My husband had a very scary situation with that stuff many years ago.

How do I get a thin film on? Duh-oh. I am shaking it, could not find a proper sprayer.

I am putting it on Hostas because something is eating them. It is not slugs, to the best of my knowledge, so I thought I'd see what the 7 did to stop whatever it is chomping on them.

Thanks for the warning - I wear coverings and a mask, and am careful to immediately wash clothing and self afterwards. When I took the Master Gardening class, the main thing they told us on poisons was: READ THE DIRECTIONS, READ THE DIRECTIONS! Good idea!

I'm sure they mentioned in Master Gardening class that you never apply any poison unless you know what insect you are dealing with. The directions will tell you which insects are affected by your poison. It seems unlikely the MG would advocate a program of dusting with poison for unknown problems to see what might happen;~)

Your hostas are probably being eaten by slugs. Try some diatomaceous earth around them. I agree with bean counter, identify your pest first, and then choose the least harmful method to deal with it.

If you're even asking, you're putting too much on.
Maybe you should look into some organic methods. You don't really want Sevin dust blowing around your garden do you?
It will kill everything it contacts, honey bees, ladybugs, butterflies, praying mantis - if you have anything like that left after poisoning your plants and soil. It will blow on you also and who needs an extra load of pesticide in their central nervous system?

It will kill or damage the immune systems of salamanders, toads, frogs, worms, the mycorhyzae that hold your soil together, everything thats supposed to work in a microsystem together. Too much poison kills everything and you will be left with just sterile soil to pour your Miracle Gro on - which I assume thats the blue stuff you probably use also.

Diatomaceous earth is also not a safe item to use in your garden. A lot of places have stopped selling it and I understand some states have regulated the sale. It is very dangerous to inhale that product and says so right on the container, it causes lung damage because it is ground shells of a calcium containing skeleton.

To control slugs beautifully, and organically with no cost of life to other organisms, you use Sluggo, or Garden Supply sells their own brand of Iron Phosphate pellets. I use it early and then again on top of mulch - I've got no slug damage at all to hostas this year. I contacted the company to ask about what it does to amphibians since I have blue, spotted, red and tiger salamanders, American and Black toads, and tree, bull, and green frogs in my garden that clean up the little critters very nicely. The folks that make Sluggo said it won't affect any amphibians.

The roses: you pick the Jap Beetles by hand, you spray any one of a number of concoctions for black spot - check Mother Earth news website or Gardeneweb archives for baking soda recipes, corn gluten, etc., or rip out the disease laden varieties and plant disease resistant.
If you absolutely have to spray chemicals, find out which ones have the shortest half life so they decompose rapidly in your soil and cease harming things you didn't intend to. And use it sparingly. Please.

I got bagworms in a 25 yr old specimen Corylus contorta last year. First I put some newspaper down on the ground below, then used a chemical spray by hand and spot-hit the worms directly. They died, I rolled up the newspaper and it went to the landfill, where at least its contained in 1 place.

http://www.chem-tox.com/pesticides/

I like your natural gardening suggestions and while I totally agree with you, I have chosen to use some poison. I do not use it lightly (okay, on the Hostas I was stupid!), and this is the first time I've ever used anything strong like Sevin.

Last year I could not pick Japanese Beetles fast enough and after they decimated the roses, they moved on to other plants that aren't even listed as J.B. delicacies. I decided this year, I'm gonna kill them.

Re: Sluggo - I do use it and I have not seen a slug. My Hosta Leaves are still developing small holes. I don't mean an occasional small hole on an occasional leave. I mean I have lacy Hostas! LOL Oddly pretty, but frustrating!

I truly feel guilty about using ANY poison and was hoping to get advice here on using the smallest amount possible to do the most good.

Back to more natural methods - do the JBs have any natural predators?

not that I know of - a bucket of soapy water maybe for the adults. My dad grows grapes and we used 5 gallon buckets with water in them and just knocked the JB's into the bucket. You can quickly move through plants and just knock them off into the soapy water. The conventional wisdom on the pheromone traps is that they do work, but you want to place them away from your gardens so that you're drawing the JB's away from the garden.

The other thing is that you could be wintering over the grubs probably in your lawn, or perhaps in your garden soil. I would invest in some Milky Spore - check with your County Extension agent for more directions - and spread on your lawn and perhaps gardens. That is a completely organic way to eradicate part of the life cycle.

I used to have some shrubbery that just was alive with JB's this time of year - I ended up pulling it out and planting something they didn't like. If you live in JB country you could just save yourself a lot of work. Aren't some of the Knockout Roses supposed to be somewhat resistant to JB? They don't touch Buddleia for example.

JB like some roses more than others, but I don't think any rose is resistant. I used the Bayer insecticide spray on the JB last year but I didn't have very many so I didn't have to spray much. You have to spray the beetles directly for it to work. The soapy water method is better from what I've heard and I might try it this year. There is a rumor that sparrows are learning to eat the JB but I haven't seen any evidence of that myself.