The flight lead needs to find and set a power setting, and leave it there. Leading FFI flights at about 3000’ MSL in my -7A with 180hp IO360-M1B and Hartzell Blended Airfoil CS prop, I set 21” at 2400 rpm. That flies me level around 130 kias +/- 5 knots. I don’t touch that setting as I lead the 4-ships through FFI maneuvers, but I do have 2 exceptions.
1) For 4-ship echelon turns, I dial back 1” MAP for the sake of #4 out there on the end of the rope. On rollout, I add that inch back.
2) For fixed pitch wingies, I set MAP to 20”.
Note, however, that your entry speed for the formations in Lazy 8s becomes critical. To lead fingertips, diamonds, and close trail into the Lazy 8s, I drop the nose slightly and look for about 140kias before pulling into the lazy 8. Otherwise, your more underpowered wingmen (especially #4) will drop out.