P1 specs say the stack height is 14mm.

Speedplay Zero says the stack height is 11.5mm with 3 hole and 8.5mm with 4 hole mounting.

Seems like you would lower your saddle height between 2.5mm and 5.5mm depending on how the speedplays are mounted.

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Brandon Marsh - Website | @BrandonMarshTX | RokaSports | 1stEndurance | ATC Bikeshop |

I put my shoe on the old pedal, crank at the bottom of the pedal stroke, crank arm in line with seatpos

Measure from the bottom of the footbed to the preferred position on your seat ( rails, top whatever ) by moving the tongue over and shoving the tape measure in there.

Transfer the new measurement to the seat height adjustment accordingly.

Depending on how much you drop it, you may well want to match it on the front, eg drop the rear 10mm then do the same on the front to maintain stack and reach measurements.

Don't over think it. Snap your shoe into your old pedals, measure from spindle center to bottom of the sole. Transfer everything over and do the same with the new pedals - spindle center to bottom of sole. That will tell you if and how much the difference is, then adjust your seat height accordingly.