I usually use comparison with similar combination too. But I use also my own test, specially for home mixed formulas. It works good for stronger solutions.

1) Cut short strip of film.
2) Immerse part (1/3 - 1/2) of the strip into working solution of developer. Let it be there for loger time, agitate time by time. It will turn to maximum black.
3) Immerse strip deeper and start your timer. Agitate as usually. Check the strip time by time against light. There are now two or three parts - black one, the second one turning dark and mayby rest of the strip.
4) At the moment the second part reaches the same density as the first part (ie. turns to maximum black too), stop the timer. This is starting point.

The idea is to find the moment, when fully exposed film reaches maximum density. More development could lead to blocked highlights, less development could lead to low contrast. But, of course, this is really only starting point.