Actually after this I realised that I could just set the detected OEM keys to their actual number.
i.e. I have 28 & 1 detected OEM keys.. so setting the actual license count to 28 for one line and 1 for the other balances it out.. It kind of makes sense because we "know" there are 29 oem licenses installed.
In our case there were another 9 machines which have oem licenses that were not picked up so setting another key up and calling it "OEM" and allocating it 9 licenses meant that we had our correct count.