Different Opinions, I see
If I am interpreting IC's insight and Jim's insight correctly, IC says max. magenta gives equal results to Ilford # 5 and Jim says max. magenta gets you to only 3.5.
IC: I am using the newer (dichroic) filters. No leaks, for sure nw I made adjustments) but it is possible I had leaks before. This may very well be the culprit.
Jim: in the past, max I was able to achieve was 3.5 - 4 (at max. magenta = 170 units). I don't think I had leaks between the lamp and the opening where the filter goes, but it is possible.
I recently took the Omega F head apart and adjusted everything, top to bottom, left to right, right to left, to make sure there was zero gaps/leaks.
I also changed the styrofoam liners, just to make quadruple-sure I wasn't losing anything because of a less-than-pure-white mixing chamber.