I don't think you can calculate actual VE, not at home anyway. It all depends on your cam, intake, exhaust, etc. You have to tell the system you estimate of VE.
For some management systems, you set a target A/F ratio, the O2 measures actual A/F, and the computer adjust fuel accordingly. If your system supports the readout, 0% O2 correction is ideal.
The Megasquirt looks pretty decent. But for those with loftier goals in mind, supercharging/turbos etc., I don't think the system can hang without much work. What I don't like about it:
+It's decribed as "experimental" right at the top.
+It's only been installed on two cars so far. Who will be guinea pig number 3?
+Switching from bank fire to sequential fire requires additional software (and hardware?) mods.
+Switching from speed density to alpha-N requires additional software mods.
+The MAP sensor is internal.
+It doesn't control ignition.
I don't know why they use the tach signal as a reference, to me, a cam-sync or crank angle signal would be much better. Who knows?
Not trying to knock anyone who's bought this system, I am just wary of it's experimental state and the fact that changes to the system require more mods when most EFI management systems have these built in already. And knowing my electronics skills, if something went wrong, they would say I built it wrong...
Owen