This is a good thread with useful info on bike fitting. I'm working on a drop bar conversion on a '92 Specialized Stumpjumper and I'll use some of the websites linked to in this thread to run some numbers.
It sounds like the OP has made up his mind to go with a drop bar conversion but the right flat bar can make for a good gravel bike as well. I like trekking bars and installed a set on a 1992 Trek 950. They're comfortable and cheap. And the set up issues disappear.
One thing I was going to suggest is that maybe it is possible to overthink this? I dunno. I set up my first drop bar conversion with a 1987 Specialized stumpjumper. I measured the seat and top tube and then guesstimated the set up. It worked out fine. I think bike set up tends to based on rules of thumb that in turn are based on lots of data points from experienced riders, racers, etc.
Last edited by bikemig; 08-26-20 at 08:35 AM.