Having now completed my pier I have some answers on this.
I found that marking out the holes, given the "instruments" at my disposal, was not accurate enough.
Geometrically, it's straightforward. Draw a 3.25" circle, rule a line through the centre then draw an arc (the same radius) centred on the opposite end of the line to create two intersections at 60 degrees either side. This is the geometric procedure for creating a hexagon in fact.
I punch marked and drilled three holes 120 degrees apart with a hand held power drill. I suspect that the marking was not accurate enough and perhaps the drill wandered a little. The result was not accurate enough, whatever the reasons.
My solution was to use the tripod head as a template. I removed the three captive bolts. One of the E clips pinged across room never to be seen again. I then clamped the tripod, inverted, on top of the aluminium plate, with centre pin engaged and drilled holes with a 10.5 mm drill bit. The bolts now fitted but one was a little tight so I drilled the holes out to 12.5 mm. The torque limit must be set on the drill when doing this, to avoid a sprained/broken wrist.
For the centre pin I drilled an 17/64" hole and tapped it with an M5, 1.25 mm pitch (coarse) thread. An M5 x 20 mm set screw was screwed in from underneath projecting 10 mm above the surface.
With either the Evolution mount or the wedge engaged with the centre pin, it is a reasonably simple matter to engage the three securing bolts.
In the image below you can see an inner circle of six holes forming a hexagon. Three of them are the marked out holes which were not accurate enough, the other three (rotated 180 degrees) were drilled with the tripod head as a template. The 3.25" circle used for marking out is not visible in the image.
Edited by Rac19, 09 June 2020 - 11:36 PM.