Converting Dayton 15" tube type to 17.5" tubeless

I've got a 25 ton lowboy with 15" daytons that I might consider converting to 17.5 tubless. Six of the 8 tires on the trailer need replacing so now's the time if it's worth it.

Accuride lists them in two different wheel offsets and they're available from a china supplier too for about $90 ea.
(page 18-20 of pdf file) http://www.accuridewheelendsolution...v.-1-Accuride-Wheels-Product-Catalog-2015.pdf

Where do you measure the offset on demountable rims?...

Converting Dayton 15" tube type to 17.5" tubeless

Converting Dayton 15" tube type to 17.5" tubeless