In my experience most Diapers are close enough to a "standard size" back when I fit in Mediums I never had to go up or down
With the exception of Dry 24/7s which are famously known as being way bigger than the size says (a Medium Dry 24/7 is like a Large in most other brands)
I find that it helps when you fall close to the middle of a Diapers listed size range (for example if a size range is stated as 34-42 and you are at 38 inches or so its probably a safe bet) that way you should easily be able to fit into it and once you find out where you want the tapes you can get a proper fit
If you are near the top of a Diapers listed size range its probably a safe bet to size up
Then again, you may need to experiment to see what fits you the best, this is where buying samples can come in handy
ABU has sample packs and gives you part of the money spent back as credit for future orders
or you can try a website that exclusively sells samples like Wearing Clouds and try a bunch of different things for what 1 or 2 decent packs of Diapers would cost
sure it's a bit on the expensive side; but if you are unsure about sizing you dont wanna order full cases and end up with something you either dont like or is way too big or too small