I also have Tourette's (throat ticks in my case), so I can give you a long dissertation on how and why nicotine works if you would like.
The short answer is that there are as many different types of Tourette's as there are people that have it, so what will work for one may or may not work for another.
Basically nicotine can cause an interruption in the premonitory urges that precedes the ticks. For those that don't know what that is; the sensation you get in your nasal passages just before a really good sneeze is a premonitory urge. So imagine that happening 20-30 times in a minute, and you know what Tourette's is like.
As for what nic level it would take to stop the urges ... who knows! For me 12mg e-liquid does not, and 24mg does.
Doctor COULD say, "yes, nicotine can help you with OCD and tourette" and prescribe it via gum or patch or something
Doctor put me on the patch ..... it did not work! The problem is the delivery of nic from the patch or gum is a steady even flow of nic which has proven to not be an effective way of interrupting the neurological pathway that causes the premonitory urge. A sudden rush of nicotine is affective.
As far as OCD, ADHD, and other like disorders associated with Tourette's, well, just what do you think about when you need to sneeze?!?
As I was a little kid in the 1940's, they just said "he's dyslexic"