Centre of the swing arm pivot to the centre of the rear spindle on both sides is the most accurate way. All this planks/box section etc is a lot of fluff setting up, measuring between to two sides of the front wheel (that's got to be straight) and is only as accurate as the lumps of whatever are straight, plus its really difficult alone!!!
The other way is to measure from the swing arm pivot to the end of the swingarm section, that's where the endcap with the bolt meets the swing arm. If these two measurements are the same you measure from the end of the section back to the centre of the rear spindle.
Both are easily done on your own, quick and accurate
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