A few times I was able to get my hands on really pure dextroamphetamine (>90% validated by a lab test), and even with a tolerance, there is no way you're consuming more than 100mg over the course of a day. 10-15mg was my sweetspot to get enough focus without getting locked into tasks and feeling wired. Although speed like that is pretty hard to find and quite pricey (~60-70€/g).
I've also realized that most of the unpleasant physical side effects of speed (palpitations, jittering, elevated cardiac rythm, anxiety, etc) are actually due to the massive amount of caffeine in it and not the amphetamine themselves (tho I'm sure there is a synergy between them). Even while feeling pretty high on pure amphetamine, my bpm only rose by about 4-5 bpms (compared to 15+ on street speed). For anyone in the same case, L-theanine works wonders to calm down the caffeine side effects, highly recommend you keep some around just in case.