TDS is a very broad reading.
It tells you how much is in your water ,but not how much of WHAT!
I use only TDS for my tanks.
It is not super helpful to have in tank reading without knowing how stuff in the tank is effecting the water.
From the tap I would say you have nice average water. Possibly a little higher then some lower also.
My tap is 350+ going to 400 sometimes.
My RO is 10 and my RO/DI is 0.
I use the meter to make my water correct for breeding my rams.
A quick guess says your water needs to be changed,but possibly something in the tank is leaching something to increase the level.
You will not come up with a good way to use the tool for predicting the need for water changes IMO. You still need to do nitrate test to determine your weekly water change schedule..750 sounds like you missed a couple or have a tight set up?