Aha, a magical online tool, it seems about right. I can only assume that along with Peter, you used it incorrectly. Garbage in: garbage out.

It needs the numbers fudged to use it for staged turbos. You have to calculate the engine size that it thinks it sees before the HP compressor. If you aftercool down to the LP intercooler output temp, that is the HP stage PR. Using the turbine tool also needs fudging.

I had a quick play with some rough fudge factors. I failed to download maps with points on, so here are the URLS:

LP
HP
Points 1,2, and 3 are 3800rpm 250h, 300hp and 350hp. The HP turbo needs the altitude fudged, so I couldn't put those on the same map.

Notice that these are not small turbos. Look at the turbine sizes the tool has spat out, bigger than the compressors. This is normal in big genset diesels, where they don't get loaded until they hit operating RPM. probably the most similar ground application to what an aero engine wants. These are first approximation results, I'd need to run some proper numbers to proclaim actual sizes. But they are in the ballpark.

If you don't understand the theory behind some calculator tool and be able to do some rough sanity checks, you are extremely likely to end up with a Raptor sized engineering error... Part of my scientific and numerical aptitude is that I sanity check my own numbers, using an alternative method. I catch a lot of mistakes doing that. Mistakes happen. It doesn't need to be exact, it just needs to be in the same ballpark. When you size turbos over a factor of two too small, you have dropped a pretty big clanger that will strangle your engine. If someone who has done similar engineering that you are attempting the first time says check your numbers, then check your dam numbers until you find out why!