There is no way to manually enter wastage figures into Loaded at this stage.
For some POS systems, if you use the wastage function on your POS it will pull through as usage in Loaded. However, the way we recommend calculating wastage is through stocktakes - your "variance" figure would essentially be your wastage. We go into a pretty in-depth discussion on this in this in one of our webinars which you can find here - feel free to skip to ~17 minutes if you'd like to go straight to the wastage portion.