so, the basic attack formula I'm using is a.atk * 2 - b.def. The helptip says defense is automatically factored in, however.

Would the above count the target's defenses more times than necessary (once)?

It's a newbie query, I know, but I'm trying to remake my old rm2k3 games into something better.

Have you tried it with and without the b.def on?
I'm pretty sure it wouldn't factor in the defence if you don't have it in the equation

Moving Pariah's Creed to VXAce?

The damage formula you write is exactly what you get.
The tooltip is confusing.

I haven't used Ace much since my current game is still VX. However, if I'm not mistaken, as Tsukihime stated, whatever you write into the formula overrides whatever may have had previously. Unlike VX where the formula was automatically built in, you can create your own formula for every skill, weapon, armor, and probably even items as well.