I'll have to try this and see how close it gets me to the times I use for Xtol and my range of films.

I have used the leader to test for potentially exhausted developer but never tried this test. Just on a point of clarification. My fogged leader before development is usually a greyish colour and when fully immersed goes black after a few minutes. I had never noticed it go through a grey stage and yet as the test involves keeping half out of the developer and matching then it presumably must?

I'll need to watch more closely in future