I run with BikeCad too and use 15mm as my bottom edge of head tube to down tube lower contact point. I simply measured a few lugs and copied the average. Like Steve I let the DT/HT angle fall where it might otherwise. (But I don't build bike with suspensive forks, those who do might need more clearance).

A side bar story. When I worked at Cyclery North, Chicago in 1985, they premitered all the DT for a 60* HT angle. This was what we set the jig at too. This was rather astounding to me as I was then using the trig sheets I had gotten during the Eisentraut class and also had a Vernier protractor, I knew DT/HT angles varied either side of 60* all the time. When I asked Eddy, the boss and frame designer, why was the angle always at 60* he replied "because that's what every good handling bike I ever rode had". It took a couple of months and a few frames for me to convince Tommy (the other builder) that Eddy was designing frames that didn't make dimensional sense and that the trig sheets I used were based on laws of mathematics and not misconceived opinions. We kept this to out selves, taking Eddy's critical design elements and running through the trig came up with the real numbers. When after the season I moved from Chicago I left a set of the trig sheets with Tommy. Andy