According to my math, assuming the box at the bottom is supposed to be a perfect cube, it should be 75.368331922% on the Y-Axis and 25.208156329 on the X-Axis.
So, you're not too far off. (Unless my math is off!)
I think it might be. I used your values on the Helljumper helmet when compared against the Halo Waypoint model rip, and there's a large amount of compression along the Y axis, as well as a difference in the X axis. I used the Z scale (height) to compare and contrast.
What I've ended up doing is exporting the model from Max with that weird cube on the bottom, and then scaling it against a spawned cube mesh in Blender. It's not ideal, but until bounding box data is implemented I can make do with manual scaling. I'd show my maths but... I'm going to be the first one to admit that working out ratios and whatnot is not
my area of expertise. Basic maths I can do, anything more is a whole bunch of 'nope'. I can post my numbers tomorrow and let you guys work it out if you like?
Maybe it's a difference between Blender and Max scaling, or maybe some biped's scaling is different? With my numbers, the box becomes a mathematically perfect cube on the models I've tested. I also tested against my own Waypoint rips and they seem to be exact so far... I'll look into what could be causing the discrepancy tonight.