Well, most ADD medication is stimulant based believe it or not.
So your theory on it working almost the same is true, BUT you are causing tremendous damage using meth.
Plus it gives you the rush every addict craves.
I suggest staying away from practicing your theory. In the not-so long run you will do more harm than good.
Your not only putting your health at risk, but you are endangering your family relationships, placing your self in a situation that could get you hurt or put in jail.
Try to stick with the prescribed medication. They DO have ADD medication that supposedly un-abuseable and contain no stimulants.
Meth over time will just eat you away until your living off the streets.
ADD and ADHD is different in everyone. Stimulants help the brain go in a strait line instead of zig zagging all over the place. Talk to your doctor if you think you may need medication. Heck, it might even keep you off the meth as long as you are not abusing it.