Tire pressure question

I wouldn't get too carried away trying to find an "optimum" pressure less than the max rated one. Not sure where the 'pressure formula' quoted comes from, never seen it before and don't believe it's that simple (it would suggest a 6psi inflation if only carrying 250lbs ??). They are accurate that full inflation pressure (65psi) will do best to protect the tire, with trade offs of poorer ride quality and potentially worse wear in middle of tread. Given that you won't wear out tread on a trailer before needing to replace because of time (4/5yrs), the tread wear argument is moot. On a TT I view the minor ride quality benefit as a poor benefit to trade against everyone's concern of tire failure.

Don't overthink it, run what the tire manufacturer recommends as max capacity rating.