I don't catch why you want to know melt pressure of different plastic. What YOU want to know is the cavity pressure to make a good part.
As you start a new topic from the calculation of clamp tonnage topic, I presume this topic is to be in relation with clamp tonnage. So, my opinion and way to work when I want to know clamp tonnage is to presume the max cavity pressure and keep a 20% of safety.
((Melt pressure x project area)/2200) X 20% = clamp tonnage need
ie : ((2000psi X 36SqIn)/2200) X1.20 = 39 tons
But the melt pressure itself can be very tricky to know. It depend at which place you want to know it (in front of the screw, in the runner, in the bottom of the cavity, in the top of the cavity) and what equipement you have (hot runner, cold runner, long deep mold...etc)
If you have a deep tight mold, the pressure will surely not be the same in the bottom of the cavity compare to the top of the cavity.
I think we could debate about this for a year long. The first question I would ask you is what's your needs to know melt pressure for each plastic family?