On my last build I intentionally put my flash hole center at .250 (1/4 inch) ahead of the face of my breach plug. I read about this in one of my books (which I now can't find the reference to the reasoning), and on shooting this rifle I can tell no difference in ignition - though I kind'a think the idea is to allow more shots as fouling builds in the breech...
A .720 IMHO sounds rather excessive to me,,, and even my .250 is - in this day and age... However, perhaps back in the day where one may have to shoot a more course powder (like the 1fg / or cannon powder) maybe it's not?
I've shot 1fg with my vent .250 ahead of the breech plug face and have had positive ignition 100% of the time, but this has always been in nice weather conditions.
Are you working from a "kit" build? If so, perhaps you need to talk to the builder of the kit.