I just took my boat out here in the Denver area and was amazed at the power lost at altitude!!! I have a Hallett 260 with a 525efi and could barely get out of the hole. I calculated at about 6000ft a power loss of 90hp!!! So... I am running a 23p 4 blade and need to go down but how much? I have a 25p mirage plus never used ,I can trade in or switch if anyone is interested.

How much pitch to go down depends on what rpm you are spinning at WOT.

A 26' boat with a 525EFI only spinning a 23" prop sounds off to me already. Do you know what gear ratio your drive is???

I have a 1.5:1 xr drive. The boat was slow out of the hole at Havasu altitude but not too bad. With the boat empty, it would max at 5200 rpm.

If your 90hp loss calculation is correct, then you should be able to use a 21" prop of the same model prop. 60-70hp is usually worth about 2" of pitch.