Hi Giles,
sounds like you're asking for the measurement between the aluminium cross members (spreaders) that go between the frame side plates and hold the bottom frame plates in place in the frame.
Measuring from the rearmost mounting hole and going forward, centres are at 0mm, 91mm, 114mm & 222mm.
In case you're interested its 52mm inside measure between the left and right frame plates and the hole centres on the spreaders to attach the frame bottom plates is 30mm.
I recently made a battery tray system for the packs in my E5 when I stretched it and went to 12S.
Will be interested to see what you've come up with. Edit: had a look at the thread on HF, very nice work.
Cheers
Wolf