It all depends on which fly line you use. Some are bulkier than others. For example, a double taper line is bulkier than a weight forward, and a nine weight is thicker than a 7 wt.
The most accurate way to do it is to select the fly line that suits the rod you want to use, wind that onto the reel and then add enough backing to comfortably fill the spool. Then strip it all off and start again the right way round, starting with the backing so you end with the fly line the correct way around. It's easier if you have a spare spool so that you can wind it on with the "front " of the fly line first, followed by the backing, then wind it back onto the spare spool and it comes out the right way round.
Hope that makes sense

PS Don't buy an Airflo floater, however cheap they seem. trust me