yep, let it go through as is, even if you fail on that one thing, it will be a free retest. even better is if you are there at the time of the test and can change the pot. screw if necessary to pass the emissions.

the problem is, changing this screw affects the fuelling over the whole range of speed and boost, so if you are leaning it out for idle, you are also leaning it out under boost which could have disastrous affects. therefore, if you do have to lean it out at idle to pass the mot, make sure you wind it back up before boosting it.

but if you're not sure what you're doing, and cannot check with a proper wb lambda unit, it's better to leave it to a pro.