guidelines for postflop raise amounts

I know there is not single, one-size-fits-all answer to this, but I'm just looking for a very rough guideline because I'm realizing I've been all over the place in how I play this and I've read some conflicting things out there....

What is the standard amount you raise by when facing a bet postflop?

Let's take a simple & pretty common example: you're in a micro-low stakes game, facing reg/abc type players with no reads, and both you & the villain have full stacks.

Preflop: Hero is LP with AK.
Folded to Hero, Hero raises to 3.5bb, 2 folds, SB calls 2.5bb.

Flop: (8bb) A K x (2 players)
SB bets 6bb, Hero raises to ... ???

Again, I know this changes depending on the villain and the board, but in a common heads up situation like this, is there general a rule of thumb? Do you base the raise amount on a % of the pot after the villain's bet (in this case 14bb), or his bet, or both? Thanks.