Make a test strip. Keep notes. Want harder? Dial in M. Keep notes. Want softer? Dial in Y. Keep notes. Move in increments of 40 to see obvious impact. Keep notes. You'll be eyeballing it in no time, fairly obvious. Keep notes. Statistically, "normal" contrast on Ilford RC is about 40M, should be a good starting point. Keep notes. No hard numbers to stick to religiously--too much depends on filter age, type of light, batch of paper, chemicals mixed... Did I mention, keep notes.
Once you get the hang of it, exposure-invariant mixing of M and Y would be the next step, but it's only needed in rare scenarios.
By the way, keep notes.