How does/can/should one knock half-round fence posts into the ground
so they end up vertical?

Is there some rule of thumb for the angle one has to start at to end
up with a vertical post or what?

I'm knocking them in with a standard 'post hammer' (not sure what
they're called, i.e. a heavy piece of tube with handles. The problem
is that, unlike a round post, a half-round one has a built in tendency
to deviate from vertical the further it is driven into the ground.

With the current wet weather we're having it's an ideal time to do a
bit of fence refurbishment but I've run out of round posts and I'd
like to use up the twenty or so half-round ones I have.

--
Chris Green