Hi Phil, welcome to the forum!
I dont at all understand your term "referenced" please elaborate. I will attempt a guess or how I would go about increasing voltage of your inverter.
If yours is a split phase unit, then you will add a few turns to each toroid and connect the new winding in series to add to the voltage of each. The polarity is extremely critical, it must be voltage additive to the original winding.
If yours is not a split phase unit, the two toroids will be connected in parallel, red to red and black to black both sharing the load equally. In this case you will also add a few turns to each. The number of turns and wire size/method /spacing should be as identical between two toroids as possible. When done with winding each, they should be compared for exact voltage output before connecting together.
Does that help? If you provide more info as to specifics of your inverter, i could do a suggested drawing if it helps. Really isnt a hard concept or task for that matter. Just label 4 wires, remove them with toroid, do again for two. Now simply wind 10 or so turns and tape down, do again for second toroid. Next using a 120v power source, check polarity and series connect before adding back to inverter this way no surprises
The result when you are done is all the original wiring of the inverter has not been changed except for these two new windings which are added in series with the inverter AC output terminals. The inverter control circuitry never gets to see the added voltage, only your appliances get to see the change. Mine runs at 120-121 on the nose for L1 and L2. Its okay but doing again would try for 123-124, seems like you always loose a couple volts before appliances get it.