Rule of 2 and 4 works decently well, but you can refine the rule of 4 a bit when you have more than 8 outs: multiply by 4 and subtract the number of outs in excess of 8.

Do NOT apply the same correction for the rule of 2 though.

So if you have 13 outs (5 outs in excess of 8), you have about 13*4 - 5 = 47% to hit over the next two cards. The exact number is 1-(34/47)*(33/46) = 48.1%. The "raw" rule of 4 would give you 13*4 = 52%. Rule of 2 says 26% to hit on the turn. Exact number is 13/47=27.6%.