DanielStone: Doesn't matter, as long as it you give it a reasonable amount of exposure, and leave it in the colour developer (C41) long enough.

I've done this kind of thing a lot.

C41 films as reversal in 1+50 Rodinal need 40c and 2 hour 20 min for proper development at box speed, plenty of agitation.

1+25 is with one level teaspoon of salt per 300ml of working solution is 20c, 10min, plenty of agitation.


But 1+25 cannot be used for reversing because it develops colour dyes, you will get 'black' slides, you would then need to fix, bleach, re-expose, Colour Dev (C41 or E6 CD, I use E6 CD and get excellent results), bleach, fix, etc.

Here is the 1+25 method described above, gives a colour neg on either C41 or E6, this is german supermarket ISO200 film, expired.



Also if you use that dilution and time - do not overexpose your C41 film - it will go shitty, you will have plenty of underexposure latitude however.