Ratios differ with product. I can't stress it any more. This stuff is dangerous and can cause serious respiratory damage if used improperly. Be smart and safe and use the respirator. Twenty bucks can save your lungs.
Urethane is a little pricey.
It goes without saying that it must be sprayed with an airbrush, so if you don't have one of those, go on to the next page. The three part stuff is even higher. You'll probably have to go to your local body shop jobber to get this stuff anyway. Buy a respirator while you're there. It takes a little coordination and a kitchen timer to shoot urethane.