You are calculating the probability for a randomly selected individual of the test to have been given a false positive result. That is different from the test's

You are calculating the probability for a randomly selected individual of the test to have been given a false positive result. That is different from the test's false positive rate, which is just P(+|M') [Where I assume M' represents that the subject had not used meth.] It doesn't matter what the prior probability is of a random person having used meth (or whatever you are testing for); it only matters how likely it is that someone who has not used meth will be given a positive result.