Yeah, I agree with MMUK. Treating them as a separate, partial fuel-up is probably the way to go. If you can purchase them separately from your main tank you'd have the details on a separate receipt which might make it easier to remember the fuel volume in the cans. Then you can enter them as a partial fuel-up when you use them.
Instead of volume I would suggest you make a note of exactly how much you put into each Jerry can when you fill them. Then as you pour them in, one or more, to the tank (assuming no spills) just note the amount(s) you added as a partial fill-up.