Thanks also Bill.

Yes, especially an "ah-ha" for me as the last couple of times I've printed I've thoughtlessly (and completely unrelated to Thornton's article) changed from doing test strips using a mask to turning the enlarger on and off (it seemed to make counting against the metronome easier) ... but then have continued to timing the full print just by counting the full desired exposure; and was starting to feel puzzled as to why what was looking perfectly good on the test strip was looking quite wrong on the full print. It's a Kaiser colour head enlarger with a halogen lamp, by the by.