That your Trailer tyres have 35 psi written on them , makes me suspect they are of the american system. Europe uses for standard load 36 psi/2.5 bar, but for 80% H/W also 35 psi is used. Then if they are ST tires , wich american system uses , the maximum load is given for 65m/104km/h...
You can fill the adviced cold pressure even when the tyre is off the ground, then after loading the cold pressure will be the same.
Or at least , that small difference that your device wont give a different pressure.
Example : filled cold spare wheel before mounting to the trailer at 50 psi...
You have to find out what kind of tyres you have and the total weight of trailer.
Total weight can be found on a metal-like plate the VIN-plate MPVW mostly called.
The tyres maximum load and kind of tyre on sidewall of it , when you are busy also search speedcode.
Normal car tyre -standard...
I registered to this forum to give you an anwer.
end 2007 I got hold of the formula that the European tyre makers use to determine advice pressure for lower then maximum load.
To make a long story short, use my latest Caravan/trailer-tyre-pressure-calculator, wich is multilinguall.
So pick...