I have been printing B&W and made quite fundamental mistake that I should have avoided. I made a print (A4) on the new Ilford Multigrade 5, First hit with a test strip and it was good enough. This was a trial before I made a full size 16x12. I made another test strip with the images cropped and adjusted to fit a 12x16 piece of paper and again it was just right.
I removed the piece of 12x16 from the packet and used the test strip time for exposure, but this time print was quite flat and under exposed. I then remembered the 12x16 paper I used was the last of my MG4 that I have. But the significant difference in grade and difference in speed puts this paper in a totally different class. I had used Grade 2 for the A4 print with MG5 but the 12x16 needed grade 3 to 3.5 to get the same contrast and another 14 seconds to get the exposure more or less correct.