What you've just described is that the 'sweet spot' of your kite is between 75% and 100%. As long as the kite is flying well without stalling, and steering well, forget about pulling on the bar for more power.

Having the kite low in the window will cause it to fly more perpendicular to the wind. It will also have less lift, allowing you to edge against a stronger kite. Both of these things translate to better upwind drive (which is good), but not more power. (And the drive itself may cause you to depower the kite.)

If you want more power from your kite that the bar isn't able to deliver by sheeting, either build more board speed to get the kite deeper in the window and gain apparent wind, sine the kite to get it deeper in the wind window and gain apparent wind, or do a combination of these.

Gfly said..
On a broad reach fully powered and planing, I dont find there is a noticeable difference in sheeting the bar in from 75% to 100%.

How should I determining the optimum amount of sheeting in for max pull from the kite?

And is having the kite low in the window near the water the best place to have it for max power?

I'm assuming you want to do speed runs? rather than loop for uptimum power for upwind.

If speed runs is the answer from your description 75% would be the best spot. Basically feel for the sweet spot of maximum speed. Any more bar in will stall the kite and slow you. Any less will not give you enough power and you wont go full pace.

Also as you speed up the wind window drifts back the faster you go. Your kite and bar position changes as you speed up. Also your down wind angle increases to. So the fastest speed is obtained by and angled broad reach that slowly curves more and more down wind.

You reach maximum speed for your equipment and the wind speed when the kite drifts back in the window where it no longer can pull you forward faster.

Gfly said..
At the moment I am determining the sweet spot by bar pressure when it feels like the kite is taut.

Are there any other indicators?

If you sheet in and the kite moves further forward in the wind window, you should be able to go faster by holding more bar pressure. If the kite starts to lag back when sheeting in, you're at the limit of forward thrust (but can still use the additional power to boost, etc). Also note that once you're holding more bar pressure you will want to then move the kite deeper into the wind window to allow it to generate the additional forward thrust. Rinse & repeat to keep ratcheting up the speed.