"I have been been reading that some states are testing for cannabis at vehicle stops. I feel absolutely fine after taking my daily dose. In fact, I feel like it clears my head (and that I drive better). But I am worried that any amount might impair my ability to not be penalized. "
This is a really interesting question. In general, it is strongly advised to not take cannabis and drive, because THC causes mental and physical impairment. Low dose cannabis, or any other medicine, may not impair you to the point of failing a field sobriety test, which is what most officers suspicious of DUI will subject you to.
If the field sobriety test is failed, you will then be given blood and urine screening tests (because the officer has determined probably cause). This won't help authorities determine when you took your medicine, but it will confirm that you use cannabis (or have used within the last few weeks).
Some counties (for instance, Los Angeles) are beginning to use an oral swab at checkpoints to test saliva for the presence of several drugs, including THC. As THC may remain in your saliva for a few days, again, this isn't very helpful for determining exactly when you took your medicine. It would then be up to a court of law to determine, based on the evidence, if you are guilty of a DUI.
The safest bet is to not take medicine if you are planning on driving. Furthermore, for legal reasons, I would discourage you from having cannabis anywhere within the passenger compartment, as this may suggest you have been taking cannabis while driving.