KISS!
If you are flying the average GA aeroplane and are doing say 120 knots, drop the zero off 120 = 12 and add 7 (19 degrees or 20 rounded up)
Once you hit 25 degrees (say a Baron doing 180) then limit it to 25.
In the Jepps (Terminal AU-5 2.1 d.) it states that turns in nil wind should be at a bank angle of 25 degrees or rate one, whichever is less.