[Hybrid_11]

Yes the percentage you get is not completely accurate but it comes very close given the simplicity. Now i could not tell you exactly why 2 and 4 works but ill try to explain the reason to why you use 4 on the flop and 2 on the turn.
Ill go back to the flush draw of 9 outs. If we wanted to figure out the odds to hit the flush on the TURN we would multiply the number of outs by 2.
If we were to miss the flush on the turn the chance of hitting on the river like i said earlier was multiplyed outs by 2.
Since on the flop we have 2 chances to hit, the turn and river by taking the number of outs and multiplying by 4 gives you the chance that the flush will be made by the river.
I hope that makes a little more sense as to why 4 on the flop and 2 on the river. Just make sure to realize multiplying by 4 does not mean youll hit the flush on the turn 36% it means that either the turn or river will give you teh flush 36% of the time
Someone else might be able to provide the reason why the number 4 and 2 works specifically but for that i cant exactly tell you i just know its been shown that these numbers come within about 1% of the actual number so its a good quick tool.