Uhgg - I feel for you!
I have been using the Tetenal colrtec c41 and can only talk about my experiences with that chemistry, but I suspect they are similar.
I have always been loosing developer - I think because I dont pre-wash - anyway - my developer level keeps going down.
I have the 1 litre kit.
After about 20 rolls I had to top up the dev with water as I no longer had sufficient to safely cover the 120 film spool. I added 250 ml to about 600 ml. not too far off your ratios. I had adjusted my times already because of the number of rolls I had developed. My change was to add 15 seconds to the dev time - I did that at roll 16.
After adding the water I did not increase the length of time, but I increased the temp from 38C to 40C - I have had no problems with my development, however my developer is getting old and thick now so I will probably get a new batch of chems any day now.