Concentra Physical Friday - Unique Situation

Hi all, have a bit of a unique situation. I have to pass a pre-employment physical for a fire department Friday, which includes passing a weight check. I'm a few pounds over, so I'm going through a water load/weight cut this week, where I'm drinking two gallons a day until Wednesday, then will cut off all water Thursday through Friday afternoon to cut water weight and make weight at the physical.

Will this affect my drug test? I used to smoke, but I'm not so much concerned with whether or not I'll test positive for THC, as I've only smoked occasionally (maybe 5-7 times) since April, and will be clean for 40 days consecutively by the time I take the test. I ordered some 15 ng/ml tests to confirm, but I'm moreso concerned that they'll reject it for being diluted. Will 24 hours of no water beforehand be enough to dehydrate myself enough to not dilute the test?

Thanks in advance, appreciate any info you can provide!