It will then be necessary to determine how many pins/scallops there will be — the minimum dimension here should be the endmill (it is left to the user to choose dimensions which will work, no error checking).

The number needs to be:

stockwidth / ( (endmilldiameter + 10%) * 4 )

rounded down — this is based on:

each unit will have:

central pin standing proud of the vertical board

a radius cut around that which allows the endmill to travel around the pin

a smaller radius allowing the endmill to return

Note that limiting this by/basing it on endmill diameter has the obvious negative consequence of precluding symmetry — probably it would be better to base on stock thickness and force the user to use a smaller endmill.