Rowdy: I'm pretty sure that the drop-keel designers initially eyeballed the raising of the seat, to locate the pilot's belly button on the centerline of the prop (or similar rule-of-thumb).
Carl Schneider went one step farther and made his front keel (with seat attached) movable up and down the mast. He then test-flew various seat heights 'til he arrived at the handling qualities he liked. Both Carl and Ernie Boyette ended up with low-thrustline (LTL) configurations. In these config's, adding power raises the nose automatically -- in some cases so much that the gyro slows down with added power unless you add forward stick pressure. Whether this is a good thing, or merely not too bad a thing, is open to debate.
But you don't need to resort to eyeballing, or guessing, or a lot of trial-and-error. The procedure for laying out the major masses so that the CG lands on the prop thrustline is the same one as used to solve weight-and-balance problems in FW planes. This procedure is set out in any FW flight-training manual.
The only difference between the fixed-wingers and us is that we do the analysis along the aircraft's vertical axis, while the FWers do it along the horizontal axis. The more complete your accounting of the various masses (rotor with head, fuel, pilot, engine, landing gear, pod, etc.), and their individual CG locations, the better your paper planning will work out in real life.
Hint: a seated human in most cases does, in fact, have a CG located around bellybutton level. I risked looking silly by testing this notion this while sitting in a plastic bucket seat on the floor and rocking back and forth.
BTW, it's inaccurate to blame the scores of PIO accidents in Bensen-style gyros on the higher thrustlines dictated by redrive engines. The Bensen bloodbath long predated the arrival of the redrive Rotax and similar setups. The peak of Bensen fatalities was, IIR, in 1967. Bensen's refusal to use an effective H-stab, slight HTL even with a 4-foot prop, and low-damping (light + high RRPM) rotor had much to do with this tragic record.