Welcome to Photrio,
@Twotone ! Great to see you're also into color processing
I agree. I would personally just round to the next quarter minute and call it good. So maybe up to roll 7 stick with 3m15 and use 3m30 for the remainder.
Essentially with these reuse scenarios in a small amateur lab, you're always pretty much off the beaten path of 'official' color processing. I suppose the Cinestill and Fuji have done their homework and have done some tests to verify how to remain within the ballpark. Which is to say, I'd just stick to what the manufacturer of your chemistry outlines.
Continuous agitation. C41 is really intended for constant agitation, so any scheme that gets you the closest to this, should be the best option. In a manual inversion tank, I'd aim for a couple of inversions every 30 seconds at least.
Here's how I usually do it:
1: Pre-rinse (essential in my experience for 120 and sheet film in rotary processing; YMMV). Also serves to heat up tank, reel and films.
2: Developer (one shot in my case). 3m15s
3: Stop bath (dilute cleaning vinegar; one shot). 30 seconds or so. Not critical; anything above 10-20 seconds will be OK.
4: Bleach (replenished Fuji in my case). 3m15. I use rapid access chemistry, so my bleach time is excessive. Doesn't hurt, because there's no such thing as overbleaching.
5: Fix (Fuji RA fixer). Again 3m15, and again this is excessive and not harmful. I use the fixer one shot at higher dilution than Fuji documentation; generally around 1+9.
6: 4 wash cycles with process temperature tap water, one minute each.
7: Final wash with distilled water and photoflo.
All steps with continuous rotary agitation (Jobo).