I think you can use samples normalized to 0db.
By default VOL at 0 lowers to - 12db (compared to +63).
Default Level is 108, - 3db compared to 127.
(-15 db by default with Attributes GAIN at 0 db, MAIN at 0)
Edit : if you prefer to export at - 6db anyway, check this (VOL=+27 = +6db) :