Iggy sez:

"I will use 3 of my 92 mF capacitors, connected in parallel, between
leg 1 and leg 3 of the 10 hp idler, if I use a 10 HP idler."

That would be about right for a 10HP idler motor unbalanced RPC. It is
close to the rule of thumb for 30 uFd per HP. It will probably make you a
fine RPC. Now, here is where a problem may occur:

You specified a "Hanrahan" Self-starting RPC. It is unlikely a 10 HP motor
will start reliably - i.e., without excessively long spin-up time drawing
heavy current and popping breakers, etc. IMO, you will need to include a
capacitor start circuit. It doesn't have to be elaborate with voltage
sensing relays, or such. Between the same line side and 3rd leg where the
run capacitance is connected, connect a momentary push button and some
electrolytic (start) caps. You will need in the neighborhood of 700 - 800
uFd. I would suggest trying 1 of the 270 - 324 uFd nom., start caps as sold
by Grainger. That would give you around 600 uFd counting the 3 92's of run
cap you have connected. If that isn't enough to start in less than 1 second
spin-up time, increase the amount of electrolytics. Never leave this type
of RPC unattended. In the event of power failure, when power is restored,
the motor would be attempting to start with insufficient start capacitance
and might burn up and/or cause a fire.

Be careful, the voltages involved can be lethal.

Bob Swinney