@3dmntbighker I have put a new version on thingiverse, 3649565. I have increased the play, and made the bolts screw in from the side. This is easy for the bottom bearing, but you need a 7mm bolt for the upper bearing to fit between the vertical bolts. Their spacing is determined by the holes on the PCB.
I have done more experiments with the gap between sensor and top bearing, and have concluded so far that a 5mm spacer is probably ok. I realised that my initial results were essentially single points for the max and min - with repeated runs, the picture is not so clear, but I think 5mm is better than no spacer.
The percentages I get are still high, but quite reproducible - for the 5mm spacer, the average is about 270%, with the min not below 259, and the max not above 287 on 4 runs of at least 500mm extrusion. The relationship between sensor distance and average percentage reported is very linear - I have 12 points in my dataset now, with a least squares linear fit r squared of 0.998. The slope is 23.0 - the average percentage increases by 23 for each mm of gap between the sensor and bearing surface.