Yes, I get that part and that is why I was suggesting this:Innit glaringly obvious - Your stud work at the back, needs to be 4mm shorter, than that at the front - by FM's calculation, but that relies on your base being the perfectly level that you said it was.
3. Put them in one by one and use a straight edge across them to ensure that it is not rocking across the joists when placed across multiple joists. This would suggest to me that they are flat and hence a OSB Board will fit flush onto this.
4. As part of #3, where a joist is hanging mid air (on stud wall end) when screwed to the wall plate, can I use packers (plastic/plywood?). Again, check with the straight edge and ensure that the joists are at the same level. This is the equivalent of my "sight test".
I am proposing that I pack the stud to achieve the difference in height as the current stud work is level.