Reel, read the manual for the tester. The web is an awful place to get technical education, but a terse article on insulation testing is
http://www.branom.com/literature/irt.html There are a lot of “what ifs” in figuring hard-and-fast insulation-resistance numbers.
A useful booklet by Biddle is “A Stitch in Time”. Comparison to other tests for similar pieces of gear is often useful.
