The bioavailability of oral Amphetamine is generally quoted at ~75% but in theory could be as high as 100%. There are many variables at play concerning how much Amphetamine is absorbed and how much is ultimately excreted. It is a drug with some unique pharmacokinetics. More acidic urine, for instance, will cause the drug to be excreted faster and in greater quantity.
I've read a lot of different information on the subject of rectal administration for both Methamphetamine and Amphetamine and I feel that the closest possible estimate would be 87%.
Whatever it is, I believe you can at least assume a slightly higher BA compared to the quoted 75% for the oral route with a faster onset.