well just souped the 3 rolls of black and white I shot at the weekend, ilford pan-f looks fine, kodak tri-x 400 at 100 (7:30 in rodinal 1:50) look great and I decided to just guess the tri-x 400 at 50 so did it for 6:30. the rodinal label said tri-x 400 at 20c was 14minutes at 1:50 (massive dev chart says 13 but hey) and by your rule francois that means it should be 7 minutes at 50 iso, but as the massive dev chart recommended 7:30 for 100 iso I decided to go a bit quicker to 6:30 and it looks fine!
one thing tho all three rolls have a slight purple tint, the ilford looks like it normally does when I dev it (very slight tint) the kodaks look quite a bit more purple, Is this my fix? I fixed the 50iso roll a little longer and it seems to have reduced the purple a tad but not a great deal. is this just how the negs of this film look?
will do a test scan when the negs are dry, will have to be a 3 strip scan and stitch in photoshop... if i can get it working today.