Measurements and math might indeed be a good way, if not the
best. Alas, it's more science than I can manage at the end
of a hike.
I sometimes start by tying the hammock loosely, without the
treehugger straps, using a taugline hitch. Then you can
adjust the hammock back and forth. Once it's centered, tie
one end the right way, but without pulling it very tight (so
as not to slip the tautline on the other end). Finally, tie
the other end and tension the whole thing at that end.
When I'm doing this, I usually gauge things by how far back
the working end of the hammock line comes along the
ridgeline. Things like "it's a forearm short of touching the
fabric of the hammock."
Or take along a buddy and help each other out.