Hello @SeriAU
The issue with the NEW FUEL SYSTEM simvar always returning 0 is tracked here:
Simconnect FUELSYSTEM simvars - [MSFS 2024] Bug Reports / SimConnect - MSFS DevSupport
You are right about the behavior of FUEL TOTAL QUANTITY, so assuming the NEW FUEL SYSTEM simvar is fixed, you would have to implement a different logic to get a consistent result.
I have logged a bug so this is explained in the documentation, and we will also consider adding a _EX1 variation of that simvar that would return consistent result across fuel system types.
Regards,
Sylvain