This is how I would calculate it:-
Have a look at another quilt top that you like the relationship of the border to the top, work out how much bigger the border is that the top as a percentage (this is only a guesstimate, we usually have borders the same width all round, but height and width different). Say it's 15%.
Then measure the quilt top and add this percentage to get the size of the top with borders, add up twice height and twice width plus a bit for mitres and overlap and you have your binding length. Work out how many WOF fabric this is and thus how much yardage it takes and subtract that from the yardage you have - even if you were out on the estimation because the useable fabric left is usually full widths it shouldn't make that much difference to how much you use for the binding.
Then seeing how much you have remaining and using a whole number near the approximate number you had earlier see if you have enough to cut the borders, being sure to cater for doing them length of grain if you wish. If you don't have enough then you go smaller, if you have fabric to spare then go bigger.
But most importantly, what do you think looks right - does the pattern mean a mitred border might look better? Use your common sense and measure everything and measure again!
Good Luck
Anne