mixing MEKP and resin, perfect amounts?

Ok, I have done a lot of fiberglassing, I still really haven't got the knack for it, I always use it in such small quantities I just mix a whole crap load of MEKP, it hardens withins minutes, I think it is just short of making it buble. I usually fiberglass pieces no larger than 1 foot by 1 foot so it never makes a big difference. I have a few things that I need to fiberglass now that are quite large. Interior panels, dash, and cover for the rear firewall (pontiac fiero)IF I mix the resin, and it starts to gell, can I mix some acetone in it to thin it and then finish the job?What is a good trick for mixing the perfect amount of MEKP and resin? I am using polyester btw.

Heh, I also just found out that thinning the resin too much with acetone, and then putting it on the felt is not that good of an idea. It barely even soaked into the felt, it just went through it, and then dripped down the inside. I had heard that thinning it would make it soak in better.. how much should it actually be thinned? I had it thinned so it was like milk.