There are various formulas and ratios suggested for "good" notches, but maybe we first should look at what a notch on a bowdrill hearthboard is supposed to do.

Some ideas:

help keep the spindle from flying away
maximize wdod-to-wood friction
deliver friction-produced "dust" to one small area
deliver fiction-produced smoldering "dust" to one small area

These ideas, if accurate, suggest:

a relatively narrow notch
a notch that does not go into the bottom of the spindle socket
a notch big enough so dust will not be trapped in the notch

Given variables in spindle size, hearthboard thickness, size of "dust" produced, ease with which dust falls from the notch, and whether or not your setup is producing smoldering "dust,"
you may have to adjust your notch several times with any given pair of spindles-and-hearthboards before things work right.

You may also have to work on the business end of your spindle to get its shape and size to work well with your socket in producing friction, "dust," etcetera.

Thoughts?