Tolerance is always bothersome, and I'm afraid at 120mg/day you will be seeing a lot of that.

If you are interesting in reducing your intake, I would suggest supplementing the amino acids which are converted to dopamine, which are L-Tyrosine and L-Phenylalanine.

For example DL-Phenylalanine (the "D" is an important addition here) can seem to double the potency of amphetamines. Supplementing magnesium reduces stress/hyperactivation in the brain, and enhances amphetamine uptake by the body.

There are also herbs that will help you, Rhodiola rosea for example is used to treat addiction to opiates and to stimulants. It also greatly augments stimulant effects; too much for some people, but doubtfully so for a tolerant amphetamine user.

Keep in mind that Dexedrine is more potent than Adderall, and additionally, one can actually be prescribed pharmaceutical methamphetamine; it is called Desoxyn.