Hello, again, Michael, (just read the previous comments). I don't think the effect can be practically solved with a UV filter because of the various conditions under which you would want to enlarge. The focus effect will shift dependent upon the filtration used, the amount of enlargement and the aperture. Also, Ctein was using specialist UV filters which only reduced the problem and cut down light by half and made grades 4 and 5 fall outside the parameters. The only real compromise, he suggests, is to make fixed focus tests with shims at 5mm and 10mm and none, under the easel to: 1) determine if you have a shift, and 2) to approximate its compensation. This being for one paper, lens, filtration, enlargement size only.
This is how I interpret his findings, anyway. (It appears that the theory of focusing through a blue filter - which I have never done- is not a reliable method).
Regards, Mark Walker.