If you don't have access to a distortion analyser or some way to see the FFT, don't bother setting P3, leave it halfway (resistance-wise). It sounds good enough without needing changes.
You can use a simple attenuation jig and the computer you're typing on to do this analysis. You need a free software like RMAA and a couple of resistors, a capacitor and a length of wire with a suitable 3.5mm jack. It's not very difficult if you want to do it, but it isn't 100% needed.
EUVL has a nice guide on degenerating the J74 source to get excellent results without complicated measurements, but since you've already soldered yours in, it might be a bit late for you.
You can use a simple attenuation jig and the computer you're typing on to do this analysis. You need a free software like RMAA and a couple of resistors, a capacitor and a length of wire with a suitable 3.5mm jack. It's not very difficult if you want to do it, but it isn't 100% needed.
EUVL has a nice guide on degenerating the J74 source to get excellent results without complicated measurements, but since you've already soldered yours in, it might be a bit late for you.