I know many prefer a tight fit of the diamond in the seat. However I
like a just slightly loose fit. The reason being if you force fit the
diamond you run the risk of breaking the girdle. Also if the stone is
just a little bit loose you can see your progress as you tighten it
and know that its not just the press fit holding the diamond in
place. Remember to drill out the hole large enough so that your stone
is not resting on the main part of the pavilion but just at the
girdle. To tighten I start with a medium point (but it depends on the
size of the diamond) carbide burnisher held at an angle so that the
metal plows over onto the stone. I finish off with a finer point and
an end brush if needed.
And if you get in the frustrating position of having the stone set
so its in there and won't come out but its still a tiny bit loose,
its cheating but, you can go in with a fine point tool behind the
stone to raise the seat a tiny bit.