Two things here. "How paint works": paint comes in usually four different bases for the different colors. Pastel base has a lot of white pigment and very little room in the can for adding colorant. Medium base has less white pigment and more room. Then there deep base and ultra base, which is nearly transparent and has a lot of room to add colorant. If you want a dark color, you have to start with a transparent base, not a lot of white. That is why ultra dark colors can take more coats and take longer to dry. Most colorants don't dry well and are not as strong as paint. (BM's new Aura is a different animal and covers extremely well in dark colors and dries fast.)Dark colors can take 2 to 4 weeks to "cure" before they stop rubbing off.
Now how your 50% equation fits into this. Back in the days when all houses were painted in pastels, a common method of picking a ceiling color was to add only 50% of the color formula to the pastel paint base at the time it was ordered at the paint store. You would have a light color on the wall and a 1/2 strength, lighter color on the ceiling. Today's colors are deeper, and if you ask a paint clerk to mix 50% of a medium base color, she or he then has to decide whether to go with the medium base or move to a pastel base. Either decision changes the equation and you will not get the color you pictured in your mind. I sell paint, and I can tell you I have mixed many 50% formulas that simply do not work, and no one is happy, because the color was too deep to use this old idea with.
I prefer to walk over to the color chips and find a lighter color by eye that will work great as a ceiling color with the wall color the customer has chosen. I have many years of experience in working with color, as well as a BA in Art from Cal Berkeley. You might be surprised to learn how many blue collar wage slaves are actually very well educated. If you can visit a few paint stores and talk to the staff before choosing paint for your next project, you may find a "free" color consultant in the process. Just be sure to buy your paint there.

Thanks for explaining that! I have a BA in interior design but I'm a new designer and they didn't teach much practical stuff in school.

I was wondering why the 50% thing wasn't working. I was hoping that I could make my job easier by taking the color I used and reducing it's value. I can see myself now, trying to find a similar lighter color and this one will be too pink and that one will be too green.

Oh well, such is life. At least I know how the paint process works.

Thanks for your help!

You can do it! You can!! I know it really frustrates people, choosing a paint color, then a trim color, wondering if they work together. And it does take time to look at the colors on the chips display and take it all in and then narrow the search. But if you are a designer it is time well spent, and the more you work with paint colors, the more familiar you will get with them. Eventually you will absorb them, the way you learn a 2nd language with everyday use.
If you want to play with paint colors, to familiarize yourself, you can download ben moore's colors into your photoshop or autocad programs, just go to the website. It does not compare to using real life color decks or paint chips, but can be an interesting thing to do in your spare time.
Over the years, I have found the brain learns things subliminally, such as looking at something large or small and just knowing what it will measure before you get out your measuring tape. You will pick up all sorts of color knowledge and it won't take that long, you will see! Have fun!

Thanks fab50 for such great knowledge.

Thank you Fab50, I didn't know you could download the ben moore paint. Good to know!

For my house, my hubby has said he'll only paint with sw cashmere (rolling my eyes). It's frustrating but I can work around it, I guess.