process in a single reel 120 tank, agitate every 15s, 2 agitations, then hold upside down, next agitation cycle, 2 agitations, hold right way up etc.

You can dilute, it's just sub-optimal results.

+50ml might not hurt it that much though tbh, I'd increase time slightly.

I've seen and tried 1+9 semi-stand (didnt add carbonate though etc) for an hour, very very grainy, but reasonable colour.