Or while being prescribed desoxyn by a doctor, Is there a limit of methamphetamine in one's system that can allow one to drive and not be charged with driving while impaired, in the state of Minnesota? Kind of like .08 for alcohol. And or if it is not impairing judgment .
Assuming you mean methamphetamine, then the answer is no. It is illegal to drive with any amount of methamphetamine in one's body.
The main Minnesota law on driving while impaired (DWI) is Section 169A.20 subdivision 1:
It is a crime for any person to drive, operate, or be in physical control of any motor vehicle, as defined in section 169A.03, subdivision 15, except for motorboats in operation and off-road recreational vehicles, within this state or on any boundary water of this state when: [...]
(7) the person's body contains any amount of a controlled substance listed in Schedule I or II, or its metabolite, other than marijuana or tetrahydrocannabinols.
The schedules of controlled substances are at Section 152.02. Methamphetamine is listed in Schedule II (subd. 3 (d) (3)).
However, there is an exception if you were taking the substance as a prescription. Section 169A.49 subd. 2:
If proven by a preponderance of the evidence, it is an affirmative defense to a violation of section 169A.20, subdivision 1, clause (7) (presence of Schedule I or II controlled substance), that the defendant used the controlled substance according to the terms of a prescription issued for the defendant in accordance with sections 152.11 and 152.12.
Note that since this is an affirmative defense, the burden of proof falls on you to prove that you had a prescription, and that you were using the substance according to its terms (e.g. taking only the prescribed dose). In particular, if your doctor or pharmacist told you not to drive while taking it, then that would seem to say that you were not using the substance as prescribed.