its easy to prove that in your head. say you have 8 outs. you're dealing with roughly 50 cards. so 8 out of 50 make you win. so %'s are out of 100, so doubling gives you 16 out of 100, per card. round up a % or two because its really 47/46 not 50 you're dealing with. multiply by 2 if you're getting multiple cards.

i find these things easiest to remember if you understand why they work.