Hellraizer: I think the only way outside a crony or app that works is to get some ballistic putty or similar that someone else has cronied against using the same/similar pellets. I remember Beeman used to sell putty and it came with a chart to estimate fps, but I haven't seen that or similar in a while. There are people here that use Duct Seal so maybe you can do a search for that? Crude but much better than your app that clearly isn't working. The app should be able to work but something is upsetting it. Maybe it's reading the shot and the shots echo rather than target impact. Or maybe the piston impact noise and shot? Or maybe your target distance is wrong, phone location wrong or who knows what. I'd check all the data settings and what not, plus shoot where there is no echo and see if it changes.
That app, or some app somewhere, will likely be able to see all noises, then from there you could ignore the wrong noises and do the math yourself. Lots of airgunners have a crony so maybe you could find someone locally? Maybe a local airgun club? Same with firearm ranges/clubs. Not sure how common those are in CA but they're everywhere in the US. If I have my crony out people often ask to take a shot over it.
I suppose another might be measuring pellet drop over distance, which is not very accurate but could at least tell if you're doing better or not.
Dukemeister: If a full power B18 is making 12 or less there is something very wrong with it. 15-18 is more the norm out of the box. My record is 20.4 for a .22 nitro version, after I tuned it. That's using all the oem parts, nothing special, no dieseling and using the spring that came with it. So while exceptional, it is possible, but not nearly as impressive as the 56 Hellraizer got
The lowest power I netted after a tune was 16.5 with a coil spring 177, which I discovered the source of the power loss was a bad barrel. Switching to another random barrel I had lying around instantly netted 18. Putting that bad barrel on another gun caused a similar loss. Imo the vast majority of B18 barrels are bad to varying degrees, so I almost always do a little work to them, or replace them.