back spacing
Where are you going to have the need for a 4500 axle .On back spacing if using spacer are adapter will need to think about that
There's some info on TNTTT that might be helpful:
I'm not sure why they make this info so hard to find on their website. I knew it was there and it took me a while to track it down.
When you calculate your axle position, due you include the tongue length in the over-all dimension? If so I'm seeing 62% back from tongue, 53% back on box only. This is factoring a 12% tongue weight. If true, then I'm very close to the 60/40 rule and can move my axle forward.
This program defines box weight, trailer weight & tongue weight (and length) separately. I do like that part of this.
Screen shot could be a bit confusing, but it gives you the idea of what I'm doing with the program.
View attachment 528079
The 60/40 guideline is based on box length.
But it's just a guideline, obviously heavy tanks and banks can be positioned to allow departing from that in either direction.
The ideal IMO, especially with axle-less suspension is a box design that lets you relocate the wheels as needed when later changes moves your CoG.
Yes, include the tongue length. It's part of the "beam" that your spreadsheet is summing the forces on. I wouldn't personally worry about being a couple percent too far back.
You don't have be dead on. you can adjust your load .Goody in side as you drive