The Schatzke's numbers remain a marker of approximate "rarity" but are of course do have occasional errors as to whether a cancel exists on a particular issue and whether it is exactly that degree of "rarity" but functions as a basis for how we view a cancel on classic stamps of Mexico. It is confusing for most collectors. I use the "Stout" interpretation of Schatzke, namely a 15/3=45 points of a cancel (15=rarity, 3= popularity of a cancel). Overall stamp value=stamp+cancel value, at 45 points times 1/5 converts french francs to US dollars= $31.50(this may not be the current currency conversion). If stamp value(NF) is $2.50 then 31.50 added gives overall value of $34.00. If on cover, add 1.5 times value here = $51.00. Updates of Schatzke remain a quest for many Mexico collectors and there are several contributing to this effort however I am not certain of the current status. You will also note that stamp dealers price based on additional factors with costs, market status, issue popularity, stamp condition to name only a few and not use the Schatzke valuation solely. I would be interested as to how other collectors use the valuations in Schatzke. RR Grace