I got my data (the 30fps drop) from actually testing 0.20g and 0.25g BBs with a chronograph. Using a calculator will give you the math for Joules, but saying the actual velocities will be what it spits out is like saying climate change is proven because a 'hockey stick' says so. Also, depending on the chrony used, you might get different results too. But try again with the chrony you (or your bleeding friend) have. AND, different brands of ammo can give you different results (maybe up to 20fps sometimes) due to the physical size (some are 5.85mm, others are up to 5.96mm), so that is a way to play around a bit with your chrony results for games and pinch the velocity down a bit.
Take a look at these results I got last year between two brands of 0.30g BBs:
SIIS 0.30g BBs
Bastard 0.30g BBs
And, you are also assuming that your gun actually DOES shoot a pefect 380fps with 0.12g BBs. What are the chances that is wrong too? All you can trust is your own testing with a proper chrony. If it's 380fps with 0.12g BBs, then using 0.20g BBs you'll see likely a 40-50fps drop right there. In which case you'll be fine.