Benches differ greatly in the way that holdfasts work in them. Thicker and thinner, what species of wood used, how cleanly bored the hole was .... whatever the conditions and whether the shaft of the holdfast is roughed up or not, the pre - clout angle is critical.
This, for instance, is too much gap.
It will hold at this but the toe will dig in and there will not be so much grip in the hole.
What to aim for is this:
The toe is clear and it will sit down flat with a single hard clout.
Also it will not work well if the visible gap is too narrow. It will sit down flat before much gripping force has been attained at top and bottom of the hole.
By the way this holdfast pictured is in 4" of Pine.
You don't need to be a blacksmith to do this adjustment, all you need is a 2lb hammer and something to act as an anvil. Simply, to increase the bite, give one sharp blow in this position:
and to decrease the bite one sharp blow in this position:
I tune every holdfast this way as it is such a precise angle, it can really only be judged in situe' when cold.
