I've always wondered where to start with remoulding a grip.
You could put filler almost anywhere and then grip the pistol.
The filler will reform to some extent, but it can easily fool you into thinking it must be a better fit than before.
You could end up with a grip that's too large, even though it seems to fit your hand.

I've just been experimenting with some Science Putty that came from the Science Museum - just a toy really.
This is weird stuff that softens easily in the hand and flows like a very viscous liquid - it also bounces if you drop it!
Anyway, I layered small amounts onto my grip and squeezed it in my hand.
The putty gradually flows into the hollows and can easily be peeled off the high spots.
I tried to keep as much of the original grip profile as possible.
This way you can easily see where any filler would be needed.

Just take a couple of photos and you will know where to stick permanent filler.
Removes some of the the guesswork.