Do a dyno run, find the torque peak, pick a max ve number to assign to the peak arbitrarily, say 95%, then scale it down from there based on torque at various rpm.

Boost complicates this.

 

Yes, technically, as ve is a ratio of the volume of outside air that fills the cylinders to the displacement of the engine. But you can also say that the engine has a natural volumetric efficiency, and with boost you're just giving it a source of denser air to suck in, essentially looking at how efficient it would be disregarding boost. For the purposes of doing calculations, well, it depends on what calculator you're using. Most of the turbo related calcs I've seen have a ve field that should be less than 100%, and an air density field.