How accurate is the grease fitting on the upper steering pin to measure driveline centerlines?
Imco steering has the unimount transom bracket that is in 1/2” increments - 34.5” up to 36”. Seems pretty crucial to get it right for geometry purposes.

What is the best way to measure? Motors are out and drives are off.

I did this install on my Formula 292. You'll love it. I measured with my drive and engine still in place... but I'll try to help.

With the drives on, I measured center of driveshaft to driveshaft of the props. Also measured center center of my saddle mount steering pins (is yours a saddle mount like mine was?) you can most likely measure this, assuming it's center to center. I also measured skeg to skeg. (I wanted to measure everything I could 10x because no one wants to drill wrong holes through a transom)!

You could measure transom housing to transom housing inside, and add 1/2 the width on each side.

Here was my install:
IMCO Uni Mount Steering Install - 292 FasTech

if you need any tips. let me know. also. boats on a trailer MAY NOT BE LEVEL. Do not take level for granted.
The install must be 'flat' with the boat. which may not be flat w/the earth/trailer. Mine measured quite level for my install. but looking back if I stuck a level on the boat and it wasn't. I might have just let air out of one side until the boat was flat for the install. make things easier. Always measure 10x. drill once

Assuming the transom assemblies are mounted square that is just fine. You can confirm that they are by measuring center to center on the studs holding the trim line manifold. Should be the same as the grease fitting.

Thanks for the replies!
The grease fittings and trim line manifold were same measurement.
SabrtoothSqrl - My 25' Checkmate had Latham steering when I bought it and I would not own a performance boat with out it now. I even talked my dad into putting hydro steering on his pontoon.