Thanks to @jkwilborn
I’ve actually been using that one. I’ve had a few attempts with the boxes.py software that I try it as a last resort. There’s always an anomaly of some type on an end or edge.
When I’ve dropped them a note, they say it’s ‘probably’ a rounding error, instead of 3mm try using 2.999mm 
The last time all I changed was material thickness and kerf to 0. It still was an unusable output.
So still looking for something that’s consistent.
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