Your analysis is quite correct Scott. That feature was jammed in just before I posted the code immediately before leaving on holiday over Easter so the gap hasn't been fine tuned. However I suspect it may be influenced by things such as the default font being used by the Text widget and perhpas by various system settings and display resolution.
A number of things such as that could be made user preferences when the code is more fully developed.
Glad to hear that you are using it for real BTW - Kudos to you :). (It's really a rather preliminary version.)
DWIM is Perl's answer to Gödel