Wright's book has a great set of appendices, one of which is a list of cartridges and pressures from the 1925 Nobel Industries catalog:

.303 Mk VI (215 gr)----17 tons
.303 Mk VII (174 gr)---18.5 tons
.375 Fl. Mag-----------18 tons
.375 Belted Mag--------20 tons

other catalog entries document the two loads of:

.375 2-1/2" Fl---------14 tons - 270 grain load
.375 2-1/2" Fl---------17 tons - 320 grain load (uncommon)

We all know there's no direct conversion to CUP or psi, but the above numbers are a good set of comparitive values.

Great project!