My particular attempt at making this work is to give on site quotes for future work, first time customers. Used to be more of a flat rate Sq. Ft. guy, but now see the "case sensitive" scenario. So when I quote new work I will make sure price is good for me. This is also a good time to be proactive in getting all the "while your here" requests considered ahead of time and ability to charge accordingly. Fortunate enough to have minimal traffic and mostly local jobs, so this works for me. I used to just give the price before I start, and they can have the option to refuse, but actually doing the quote ahead of time makes things flow a lot better imo. Also the tech isn't on the clock when I quote now. I have done plenty of quotes that take 1/2 hour, on payroll, to answer questions, measure, price or they feel your rushing. Then miss things (as customer talks to you) or run quick numbers wrong and you have to deliver.
So in short, pre qualify without tech on clock, make the $ for the job work for you, and have at it.
Also tracking how much gross $ taken in on 1 tank of gas. Just sayin it is still pretty good margin, but feel the pain.