That kyle, is a very good question. On Usa Kilts site it is recommended that you may want to go up a size. I went up from 44 to 46 as I has ordered jackets from the UK in the past and they were a firm fit. I guess I am a true 45, so if I want something, "Off the Rack " so as to speak, then it's a comprimise either way ( USAK jackets are supplied by StKilda and a 44 that I ordered several years ago from Stkilda was the one that was a firm fit)

If you are 42R with jackets made in the UK in the the past and a 42R with sports jackets made in the USA, I would probably go with the 42R.

I would suggest you chat with your supplier to get their idea on the sizing of the jackets they sell

All the best with the purchase