As per the official information from ILFORD (found here) the powder developers PERCEPTOL, ID-11 and MICROPHEN can be used as a stock solution or in dilutions of 1+1 and 1+3. I'm confused as to how many rolls can actually be developed with the dilutions, assuming those are used as one-shot developer.
According to the documentation PERCEPTOL can be used to develop 4 rolls of 135 (36 exposures) or 120 film per litre at stock strength. Used developer is to be poured back into the bottle to mix with the remaining developer. Suppose I use a Paterson tank where one 135 roll requires 300 ml of liquid and 2 rolls require 500 ml. I take it that would then let me do 4 developments of a single roll or 2 developments of 2 rolls at the same time, increasing development time after reuse as documented. Is this correct?
So let's say instead we use a dilution instead and only use the diluted developer once. At 1+1 I'd need 150 ml of stock to get 300 ml of developer. This would yield approximately 6 developments of 1 roll each at 300 ml (assume the remaining 100 ml stock isn't used). At 1+3 I'd need 75 ml of stock to get 300 ml of developer. This would yield approximately 13 developments of 1 roll each at 300 ml (assume the remaining 25 ml stock isn't used). Am I correct in this assessment or making some mistake in reasoning?
I find things get more confusing if 500 ml of developer would be made at 1+1 or 1+3 dilution, which in a Paterson tank suitable for 2 rolls could be used to develop 2 rolls simultaneously. At 1+1 you'd need 250 ml of stock to get 500 ml of developer. That should be enough for 4 developments, but with 2 rolls at the same time. Meaning 8 rolls total. At 1+3 you'd need 125 ml of stock to get 500 ml of developer. Enough for 8 developments, for 16 rolls total. Am I making a mistake in assuming 500 ml developer at 1+1 or 1+3 dilution can simply be used to develop 2 rolls simultaneously? Should the development time be increased in this case, as with reusing stock strength after each roll?
EDIT: as an added thought, for ID-11 or MICROPHEN this could put 1+1 dilution even at a disadvantage. Those developers can be used for 10 films per litre at stock strength. But if diluted they must be used one-shot. Then it would still only let you develop 6 single rolls at 1+1 (300 ml developer using 150 ml stock per roll) and 8 rolls in pairs at 1+1 (500 ml developer using 250 ml stock per 2 rolls).