Go for it. I thought it was a very well written letter but then again I can't take the perspective of your employer. There's a chance that your boss may be one of those grown up in a household totally opposed to drugs, the type that won't even listen. It's 100% worth a try but I wouldn't mention that you use it. It is implied when you're writing but you don't want your employer to just disregard you as a stoner. No offense to anybody here but stoners don't get that much respect when it comes to important formal things.