I free-styled mine as you can see. I had no calipers or anything at the time - just a marker pen and a ruler.
My rear brace went out wider and came back in onto the two posts. I think it would always be too flexible this way. Hupo and Joern used a carbon battery strap down the centre of the chassis - with cells either side.
It has run - I will rebuild it one day with some changes.
haha, you can see where I cut the rear bulkhead tabs with a hacksaw whilst on the chassis to move the cells even further back. 410 has never been a rear-traction-king.