ATM: Keep Polishing or Back to the old Grind?!

[Adam Rosenberg <adamr@synopsys.com>]

I recently finished polishing and started figuring my first
mirror(10" F/6).  Initial readings told me that in zone 3 on
a 5 zone mask I was out by 1/.2 wavelengths.  I've spent about
4 hours grinding out the outer half of the mirror to bring it
down to the hole in the center and now have readings of 1/.4
in zone 3.

Here's the question, am I that far out that I'll make better 
progress going back to say 220 grit or does it make sense to
put some more hours in figuring this thing.  I've seen too many
past threads where people spend their life trying to polish out
something they could have ground out in 8 hours.

Thanks,

Adam R.