The average commonly used preM do little to nothing for dandelions and other broadleaf weeds, Hogan. PreM is mostly used to prevent grassy weeds like crab grass. PreM should be dropped just before these (grassy) weeds start to germinate at about 55 degree soil temps. Use your neighborhoods forsythias (yellow blooming bushes) as your visual clue and drop preM just as the forsythia start to bud/bloom - which happens at around 55 degree soil temps. [keep in mind with preM, it is better to be too early rather than too late] A healthy, thick and taller stand of turf goes a lot farther in preventing broadleaf weeds from germinating. You have a new lawn - water deeply and infrequently and mow tall to allow for good root growth before the heat of summer comes to your renovation. Your new lawn will be hungry its first year also. Allow the renovation to wake up naturally. Mother nature does this all by herself with the warming effect of the sun. Once to start mowing grass at regular intervals, give the lawn a good feeding then. A week later, any new weed growth will spring up from the fertilizer. Spot spray as needed now because weeds are best killed when actively growing and the recently fed grass will fill in the weed's void.

To better understand why/when/where to apply fertilizer, you'd be better to read on what the individual elements... N..nitrogen, P. ..phosphurus.....and K..potossium (potash) does for plant life including lawns, trees, shrubs, annuals, perennials, and bulbs. In a few short words, nitrogen greens things up, makes them grow and can interact with how phosphurus works...which is usually to produce strong rooting and encourages flowering where applicable. Nitrogen can, however, cause the phosphurus to not produce flowering, the plant is being told to produce foliage at the expense of flowering. Potassium aids the plant to produce strong rooting and gives winter protection. Then the other elements kick in..zinc, molybedunum..(sp), copper, lead, etc, etc....you can read more about them in the fertilzier article. Far from not applying, nitrogen in spring advances the greening up and helps lawn grasses fill in bare spots; it promotes strong growth. Where flowering plants might be near such lawn, its best to hand spread the fertilizer so it doesn't act on the garden plants. This is not to say that fertilizer for flowering doesn't have nitrogen in it....it certainly does, but at a much reduce rate. Look at the famous brand of fertilizer for flowering...Miracle Gro...15/30/15...the phosphurus is much higher and promotes the flowering and rooting. Some people use the individual elements in their fertilizer...this is alright used for what they do...Ammonium nitrate has nothing but nitrogen...superphosphate, 0/20/0 and triple superphosphate 0/46/0 has nothing but phosphurus and potash fertilizer can also be bought in its only form too. To not apply fertilizer on a lawn is inviting poor growth, disease entry, and otherwise a lousy lawn. Some might suggest you apply nothing but organic matter....and that is fine, where such can be injected into the lawn and the time interval to wait for it to work. Organic fertilizer must work its way through the soil and be the stuff that the lawn needs...N, P, K...and trace elements... Inorganic fertilizer works immediately, over long periods of time and has the trace elements necessary for good soil health. Too much of any good thing however, can cause problems much the same as too much of the bad. Read articles on 'fertilizer'...and read further articles on "compost" and understand better the why's before the 'whens'.

I do not know about your area but thought i'd give you my opinion anyway! Your local extension office should be able to tell you the usual timing for applying crabgrass control. Then if you consider using the chemical Dithiopyr (one brand name is Dimension) timing is not as important. This product works over a longer time and will also kill small seedlings.

Solve the equation by selecting your "m" and it will result in the dosage amount per gallon which would cover 1000sqft. This is a common misconception. The amount of carrier (water) per ksf is a function of the sprayer volume, not a pre-determined thing, which is why spraying requires calibration. What you do is you mark a 10x10 section of driveway , take your sprayer and a timer, and see how long it takes you to spray that 100 sq ft section. You then take a large measuring cup and spray into it for the same amount of time to see how much water it used for 100 sq ft. Multiply that by ten and now you know how much water to use per thousand sq feet. My backpack sprayer with the fan nozzle puts out 1.3 gallons per ksf. The round spray nozzle puts out 0.7 gallons per ksf, but can be adjusted. If I'm trying to apply prodiamine to 3000 sq ft with the fan spray at a rate of 0.5oz per ksf, I would add 3.9 gallons of water to the sprayer and 1.5oz of prodiamine. If I'm going to do it with the round spray nozzle I would add 2.1 gallons of water and 1.5 oz of prodiamine. The amount of water will vary based on nozzle output and calibration. Calibration has to be done every time to ensure correct application.