catalogues are printed in America by an American company, so they use American grading standards; in theory, an "EF" in Krause
for a world coin should mean the same as an EF for an American coin. Though sometimes, the valuations supplied to Krause
by their informers and contributors are based on local grading systems.
As for the question, "What to do about coins in "in between" grades", if you don;t have a specialist coin catalogue from that country, then the only answer is to wing it. If I particularly need to assign a book value to an aUnc coin, I take a guess at about 1/3 of the distance between EF and Unc for a typical circulation coin series, perhaps halfway for NCLT or other series where Unc coins are relatively common.
Or, if you want to get all mathematical, plot the prices for VF, EF and Unc as a curve on a graph, and interpolate a price on the curve halfway between EF and Unc. Not sure the number you'd get back would be much more reliable than a guess.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis