Randoms can be beaten with substitution but you need to know how they give the tests so you can plan for them. You need to find out how much notice you are given and then if they sit you down in a room or let you keep working or whatever. See how long it takes you to heat up 2 ounces of water from room temp if you don't have QF. So either find out from someone who is cool telling you every detail or wait till your first random and pass it naturally and the decide if you can pull it off.

As far as how long a hit takes to pass your body and these pot calculators, I don't know. What I do know is if someone is completely clean a one time high takes about 5 days to pass a test for most people. After that, it's hard to say how long it will take to get clean.