What exactly is OBD Calculated Load?

Do you have a source for that formula?

Is it possible that you are reading Calculated engine load value, which would indeed be zero when you are in neutral?

• Range: 0 to 100
• Units: %
• Formula: A*100/255

I did find this page which defines it as:

Calculated Load Value -- Indicates a percentage of peak available torque. Reaches 100% at wide open throttle at any altitude or RPM for both naturally aspirated and boosted engines.

In which case, I would still expect it to return non-zero values when in neutral, as it's not transmission load.

It looks like you're getting the formula from https://github.com/Knio/carhack/blob/master/old/can/pids.py, and if that formula is indeed correct and implemented correctly by the car's computer and manufacturer, then it "Reaches 100% at WOT at any altitude, temperature or rpm for both naturally aspirated and boosted engines", and should indeed be independent of vehicle speed or gear engagement (or lack thereof).