I looked into a custom frame through an Italian supplier. They only talked about front center, nothing about head tube angle and rake.

I couldn't get my head wrapped around that, since I had a variety of poor handling stock bikes. I had an idea of what I liked from each one but couldn't express it in front-center (still can't). To me front-center was and is a result of the 3 sides of the bike - seat tube angle, top tube length, head tube angle + fork rake. Going the other way I can't quickly tell what happens when front center goes out, say, 1 cm.

Ultimately, even though the custom cost was nil, I decided against getting a frame through them. For one thing they said that a 55-56 cm top tube was ridiculous.