When loading a fly reel with backing one thing that you should strive to achieve is to get as much backing on the spool as possible to help reduce any memory that may occur in the fly line itself.
There is nothing worse than winding on your new line and finding you have too little or too much backing on the spool.
So when winding on the backing how do you know you have the right amount?
Well one simple little trick is when you think you may have the right amount of backing loaded, carefully remove the fly line form its plastic spool whilst leaving its retaining bands intact and then offer it up onto the spool to see what finished level you have.
Once you have the desired level of backing on the spool, simply return the fly line to its spool, remove the retainers and wind on.