Shoot, now you have me second-guessing my final print exposure method.

I make test strips in one long time exposure. I expose a sliver of test strip and uncover strips throughout leaving the "5" second strip for last... Because my light source is uncontrolled fluorescent, it is most likely to start dim and brighten during a 40 second exposure. So... If I were to follow your advice, I'd have to leave the timer at 40 seconds for the final print with the lens covered until the selected time on the countdown. Then I should uncover to expose paper for the remaining time on the timer... Or I should revise my strip exposure method to give the 5 seconds first and cover up small sections of the strip as time progresses - the way all the references say to do it.

Now I know why.

But in practice, when everything is dry, I don't find THAT much difference between the selected test strip compared to the final print.