IMCU measures initial clotting in 40 mins at 35C per the IDF 157 standard. In cheesemaking, we typically do not cut immediately after clotting occurs, but rather use a multiplier. The multiplier is the amount of time it takes for the initial set, multiplied by a factor we want so that we control the gel strength, and therefore moisture target better. That multiplier for most cheese is 3x-4x, which is exactly in line with manufacturer recommendations because we typically want the initial set to happen in 10-12 minutes and the total gel to be ready to cut in 30-45 minutes for hard continental cheese.
for the proof, don't know what you need. Do you want the IDF or ISO standard for IMCU calculation?