There are a couple of other issues which relate to splitting and border calculations, and I'm not sure if this is related so I am including it as a separate issue.
The parameters I use which seem to be getting a plausible answer:
- Build plate split mode enabled
- Average bases over plates NOT TICKED
- Build plate width/length 256mm
- Grid mode cells
- Sizes 42mm/42mm
- Grid 7/10
- Minimum base width 316
- Padding far
- Minimum base length 427
- Padding near
- Subsequent settings at default
This generates four plates, with a 22mm border down the side and a 7mm at the front which is what I expected.
If you now set Average bases over plates TICKED, the result is that the side border is enormous. I actually printed one of these before noticing and measuring it, it looks like there's a whole 42mm grid cell of extra solid side border in this case.

There are a couple of other issues which relate to splitting and border calculations, and I'm not sure if this is related so I am including it as a separate issue.
The parameters I use which seem to be getting a plausible answer:
This generates four plates, with a 22mm border down the side and a 7mm at the front which is what I expected.
If you now set Average bases over plates TICKED, the result is that the side border is enormous. I actually printed one of these before noticing and measuring it, it looks like there's a whole 42mm grid cell of extra solid side border in this case.