Patterns are built for a b-cup. I'm guessing you are an A or AA. I have a 32" bust (28" underbust), 25" waist, and 36" hips, so I usually make the size 10 or 12 so that I can breathe and the seams won't pop over my hips, then I try to alter as best I can so that the boob part fits. On my corset top for prom I had to take 4" off the top, which is why it is good to make a muslin or test piece first so that you know how it fits before cutting into your good fabric.
Also make sure that you are using 5/8" seam allowances. For some reason not everyone is used to sewing with these seam allowances, but it should say in your pattern guide what seam allowances are used.
I am between a 32A-B, I have been fitted for 30B but that size rarely exists. But in the picture with that bodice I made, it certainly looks like I'm AA cup since I'm swimming in it! So are all patterns drafted for a B cup, regardless of if you're sewing the size 8 or size 16? I really think I'd need D's to fill out that bodice though, and I made the whole top part to the smallest size.
I use whatever seam allowance the pattern is written for, usually 5/8ths like you said.
So if you alter your tops, do you just cut down the patterns and make a muslin to see if the adjustment fits properly?