I've found blades that'r too small rip through the water without letting you
get proper drive ..... bit like a car having a slipping clutch - it does go
forward but wastes a load of power.

At the other end of the scale too big a blade just digs in and grips so
tight there's virtually no give at all except a tiny bit of spring in the
shaft. That sounds OK but it really loads your shoulders. I tried a set of
DB RVXs and found I got good accelaration but my shoulders took too much
hammer whilst the slightly smaller RVX accelerators were way nicer.

I can't see how there'd be a formula. Too many variables key ones including
your weight, size, strength and the quality of your joints.

Nidge