thanks for the info
i found the following recommendation in the selden mast on-line catalogue (page 31): "(if) the cutter stay is located 3 - 6% of the height of the foretriangle below the existing forestay ... (then) ... running back-stays are not required"
www.seldenmast.com
since selden do not reference this to a particular spar section or rig design i guess it's as close as we'll get to a "rule of thumb" (given it comes from a manufacturer's documentation i suspect it is conservative)
the spar on "karen m" is from selden