I want to reconsider my first reply to the OP ...
To recap, I suggest 2 methods, or a blend there of, and the applied results to be monitored with a tire gauge [ON EDIT: tire tread depth gauge] :
1) match the vehicle placard values per the doc linked in this thread, as prev posted. Likely the placard psi posted for the OE size will calculate out to a higher value than the respective GAWR. This overage I'll call the load reserve. This seems to vary quite a bit for different vehicles. In rubberleg's case it works out to 141% FR / 125% RR (my calc from data given). For my truck it is 107% FR / 103% RR. The auto mfrs have their reasons for this - I don't pretend to know them all. (fwiw tho, I'm not certain that the construction and target application of a LT tire would motivate the same reserve allowance as a P metric might.)
If the vehicle axle weights are within the GAWRs then the equivalent mfr load reserve is included in the matched psi for the new size/type, per doc. If actual wt is higher, increase the look up wt by the load reserve % and find a new psi using the tables.
2) use your actual axle wts to find min required psi for your size, per the tables. This is minimum psi, can add more as desired, eg. perhaps 4 psi for highway. NOTE: after some reading I believe 35psi is the min suggested for LT tires.
Long story short - by my guess - this could put the OP at 35-49FR / 42-49RR. A broader and higher range than my first post implied. YMMV
Final Word. There lot's of info online. This comment I found interesting, proving me wrong once again. source: Nitto TB
"LT tires do not offer any benefits of being "heavy duty" when under-inflated."
Edited by klahanie, 07 February 2019 - 10:20 PM.