Using the weld face width allows the inspector to use the same criteria for fillets with unequal leg dimensions and to check multi-pass welds where each weld bead is subject to inspection using the same criteria.
The fly in the ointment is how to actually measure the convexity. I have a tool that I designed and I am currently doing a patient seach to see if there is anything similar. So far, nothing shows up in my searches.
Best regards - Al