A lot of folks use 40:1, especially with aftermarket performance accessories. A stock Tomos A55 uses 50:1. A gallon has 128 ounces, so 128/50 = 2.6 oz of oil per gallon of gas at 50:1 (your oil pump should be putting about this much in).
At 40:1, 128/40 = 3.2 oz / gallon.
If you want to run your pump and add some pre-mix, you could use 3.2 - 2.6 = 0.6 oz per gallon to get about 40:1