ADHD scripts are usually written to be take on a consistent basis (e.g. I am prescribed dextroamphetamine, 5mg, t.i.d.) But in practice patients are often advised to take "drug holidays" so as to prevent the buildup of tolerance and of general unpleasantness that can come from taking them on a daily basis. For me, I don't take them on weekends unless I am trying to do a lot of writing, but they are essential for the work-day. But I do try and not take them every day that I work, for the very reasons mentioned.
You are so right about the "balancing" aspect. Too little, and there is not the desired effect, and in fact it can be a bit distracting, not the least reason for this being that there is the urge to dose higher. Too much, there is irritability, insomnia, paranoia, anorexia (that is, in the non-specific sense, loss of appetite), &c. And insomnia and anorexia compound irritability something fierce. For me that is the largest problem, probably.
The thing about amphetamines, though, is amphetamine abuse is such a different animal than therapeutic use. The doses used by the typical meth abuser are higher by an order of magnitude than the therapeutic dose ... realize that methamphetamine is available by prescription in doses no greater than 5mg, and dexedrine 10mg (IR) or 15mg (XR) ... although an application to the FDA was recently approved for 20mg and 30mg IR dexedrine tabs. If I recall correctly the maximum dose in the PDR for dexamp is 60mg/day, I forget what it is for methamp & mixed salts. But none of these amounts even approach what an "average, recreational" dose of methamphetamine would be, in terms of hooving a line, smoking a bowl, or rigging up a shot of the stuff. The effects and the pharmacology are different up at the higher end of the spectrum, and above when I'm talking about the bad effects of too much amphetamines, I'm only talking about at clinical levels ... at the higher end of recreational doses it's just astounding in terms of psychosis, bad physical effects, all of that.
Properly managed I'm convinced that amphetamines can do wonders for your mood, motivation, concentration, also to treat anergia, anhedonia (one of the few drugs to treat these aspects of depression with any real efficacy). But the proper management is difficult, even if you're not "abusing" them "recreationally", per se. And if you are, forget it, you might as well be using a different drug! Only ... it's the same drug. The one piece of good news in that, I guess, is that the TI is really high for amphetamines, much higher than any other "hard drug" I think.
For the record I like meth, but I would usually take it orally, in smallish (5-20 mg) doses or occasionally IV or smoke it. But for my purposes (utilitarian, workaday stimulation and aiding concentration) dexedrine is much better, and now that I am prescribed it, no reason to toy with meth. Methamphetamine has a flightiness, a rolliness to it, it is too euphoric and prosexual, too manicky, all these things get in the way of trying to get work done. And as far as fun goes I think cocaine (although I haven't seen good coke in years) or mephedrone (although talk about pro-sexual!) beat it hollow, but as far as getting my jollies off, stimulants were never really my bag.
Sounds like an urban legend about how to make drugs ... except it isn't ...