To eliminate the cast iron table and the insert plate from the equation lower the blade below the table, move the fence in position to make your cut, then clamp a piece of dead flat melamine or mdf securely to the table, kissing the fence and raise the spinning blade up to the desired height and make your rip cut.
If it still comes out crooked with the blade at 90* and the fence parallel to the blade it might be something not necessarily saw related.
I have 2 machinist squares that are not dead on 90*, one that I dropped and one that I found out the hard way, just saying.