There are few things to consider when calculating drug loading efficiency
1- Whats your starting concentration
2- What mechanism are you using to remove free (non-loaded) drug?
3- What mechanism are you using to quantify the loaded drug?
For example, if you are using liposomes, and loading a fluorescent drug, then you can use Dialysis method to remove free drug. And later on you can use microplate reader to quantify the loaded drug, provided that you have a standard curve for your drug fluorescence.