It's only accurate if the data you provide is accurate. Garbage in = garbage out. There is no way to what your VE is at what particular RPM, and who here knows what their BSFC is...
I prefer the hand calculations on Garrett's website.
He's trying to say you have no way of knowing what BSFC our cars generally run and there is no way of knowing the VE of the engine at a particular RPM. In fact the VE and BSFC numbers in the table look like they are not even close to accurate as-is.
If you put different numbers in for VE at RPM or different numbers for the BSFC it will drastically influence the resulting compressor map, and since there is no way to know the true numbers then this calculator is really only useful as an academic tool. It is pretty cool, but the results you get out of it will almost certainly not match real world results.
You will get much more accurate results by plotting the known airflow of our engines on a compressor map by hand.
I played with this calculator for a while but never was able to get it to work for me. I don't know the exact numbers that it needs to be accurate.
To the OP, if you're looking for a bigger turbo, I've got a VF8, injectors and intercooler in the FS section right now. Its no Holset but its larger than TD04 or VF10/11 and works with the stock fuel system.