Going back to post #11, I showed a spreadsheet with little explanation. Here it is again
First assumption is that it doesn't matter about the total length of any wire, but only about the relative lengths. If you get one wire the right length, you can calculate how much longer or shorter the other wires should be to follow the same natural curve.
I arranged the wires to come out of the PSU is a certain order, so that they wouldn't have to overlap or cross each other on the way to the motherboard. You can see at the bottom of the spreadsheet that I put 5V red wires in the middle, and 3V orange wires on the left side, etc. Once the wires were bundled up, they make a square. Each wire in the bundle would have to turn 180 degrees to reach the top of the PSU. The outside wires would make a longer turn than the inside wires. If you imagine the distance between each row of wires as an increase in radius, then you can imagine that radius x pi will give you the delta of relative lengths for these curves. The values are in rows 5&6.
As the wire bundle continue on to the top of the PSU, they need to make another turn. This one I simplified by splitting the bundle in two: wires destined to the lower row of the ATX socket, and wires destined for the upper row. Again thinking of the difference in radius allows you to find the length of the curve.
If we look at the socket on the motherboard for the ATX plug (pic below), we can see that each of the wires makes a similar shape, the different is how far along the plug they are. I assume that because the pitch of the pins in the plug are 4.2mm apart, so the wires must be multiples of 4.2mm longer depending which pin they must reach.
Then I picked a reference wire: the bottommost 5V and that must be the longest wire because it starts at the bottom of the bundle out of the PSU and it goes far out over the motherboard. All other wires are cut shorter and will be routed relative to the reference wire.
Once the wires are adjusted and the clips screwed onto the PSU bracket I finally saw the result of the preparations. I made little adjustments at the PSU end. When satisfied I tied up the PSU wire bundle with zip ties and pushed it inside the PSU so it wouldn't be visible.
It's not perfect but with a bit of dusting and some photo effects it looks nice. The USB3 wire I eventually moved so it was more at the front of the case.