How do i find out the volumetric efficiency and whether to calculate as a 1.3 or 2.6 fuel wise.

 

Volumetric efficiency (usually stated as VE or Ev on this forum) will depend on the type of porting, turbine efficiency, exhaust back pressure, and other factors.

The standard formulas that you will see for engine airflow will assume a 4-stroke piston engine which only cycles half of its displacement per output shaft revolution. In order to compensate for this, double the displacement for rotary engines and 2-stroke piston engines when using the 4-stroke piston engine equation.

More information on turbo sizing for a rotary engine here:
https://www.rx7club.com/spec-tuning-154/how-size-matters-478915/

Fuel wise, the rotary engine tends to eat a lot of fuel, and the people on this forum tend to like high boost yet have poor tuning skills. Turbocharged rotary BSFC tends to be in the .65 to .75 range, depending on the quality of the tuning, boost level, etc.

Also, you can post in your car's generation-specific forum to make sure you have chosen correctly prior to buying anything. There are many opinions on this forum, but at least you can make sure you are in the ball park.