Not answering you question, but in 23 years I have never argued warranty, if it was wrong or broke I just fixed it. Had one customer calling complaining a relay fried after 3 years, I said chill, I'll replace it no problem. He was astounded and insisted on paying. I still do work for this guy 18 years later.
If you and your men do quality work there should be few call backs.
Things do go wrong sometimes, fixing these should be a small price for a good name in the market.
Stand behind what you do.