Ok, maybe this will help.
My 19" veemans are 8.5j et35 with 235/35 tyres and I have no spacers fitted.
If you were to compare an 8j et43 with the same 235 tyre plus with a 15mm spacer it would be ET28, which for comparison terms is only 1mm more than what my veemans are on which would be be like running ET29 on 8j .
so yes 15mm is doable but that would be a max if it were me.
keep in mind the 235 has a small stretch on an 8.5j rim which helps if it didn't then it would be more of an issue and would look like a tonka toy.
I had to go et35 8.5j to clear my 18Z's which they didn't but it wasn't much to overcome.
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