You have prompted some more thoughts, and I have discovered how to find the answer.
By using
@media screen and (min-width: 1024px){…..
and setting some custom format parameters for screen sizes above that size, I can vary the number of pixels (the 1024) and see when the format flips between my custom format and the standard one for each of the three options.
I guess that maybe this is what Steven meant.
Thanks for taking the time to help.