Mike, Thanks for the prompt reply. This is actually for an adaptation of the spot welder circuit you gave a link to a few days ago. That circuit used an adjustable voltage regulator. I thought it would be at least as easy and provide more charging current to use a P-channel mosfet controlled by the comparator. I have a crude concept circuit working. I will post the completed circuit when available.
To answer your question, I don't think tolerance will be a big deal. The TIG and MIG welders I have used simply had a pot with arbitrary markings for "heat" or its equivalent. I will be doing the same. Reproducibility is more imortant than the actual voltage value.
Also, with the comparator, I can create a delay to have the relay disconnect occur a little before firing the SCR to be sure that part of the circuit is protected. John