I would convert the 5/8 and rest to inches and then divide by 36" to get the total yardage. For example 1/8 is 4.5" times 5 is 22.5" then add 1/2 yd to that which is 18" for a total of 40.5" or 1 yd 13" etc.
And yes, you can do the total and not have to cut the 5/8 first. However, read the instructions. There may be a reason to cut in 5/8 yds for ease of cutting the sub units. It might make it easier to cut in multiple layers.