Had a cunning plan, just add a 3 tone synth to get a consistent volume. TLDR: when adding split merge, set the main level of the merge block to -3db
Here's what I did
1) Add 3 tone, default settings. Gives an steady output of -17.9 Db RMS
2) Reduced level of 3 tone block to -3db just to sanity check and we get -20.9 Db RMS. Ok that works, 3db reducation as expected. Leave it there, so -20.9 is the target.
3) Add a split and a merge block (note that the merge has a +3db default) = -14.9. So yeah that's 6db increase with the default +3db
4) set the merge block to -3db, a total of 6db reduction and we're back to -20 Db RMS.
So there you go, answered my own question. When adding a split, just set the level to -3db which makes sense as 3db is a doubling of volume
Why they set a default boost I dunno, I reckon they forgot a minus sign!