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I think the distance of the saddle relative to the BB should not change. This position should be determined by your body's center of mass. You want the weight roughly balanced between handlebars and saddle correctly. (Conventional wisdom is 60/4o I think)
Ignore the seat tube angle, you can compensate for this with an inline / layback post and moving the saddle along the rails.
mind_maze-
From a fast road position? Does not compute. How are you going to maintain your arbitrary 60/40 with the same effective ST angle and the longer front-center of the gravel bike? Are you going to lower the rider over the front end?
Moving from road to off road positions usually become less static because of different terrain. That’s reason to look again at all your road measurements. Of course a good starting point is probably your road measurements on a frame with some further possibilities for adjustment.
miro_o
Thanks, that is really helpful. I notice that if reach stays the same, effective TT is longer on a gravel bike. I've always gone with reach/stack, but with the saddle further back (due to seat tube angle) does effective TT become more of a factor?