I don't think that small amount of fluctuation is much too worry about, adjust for the highest reading and back off a tad until you can time it with a light for more accuracy.
4gauge so be plenty large enough if the connections are tight, though that's almost 4 feet of length. Better would be "o" gauge welding cable. I once had a 72 Caddy that had a 4 foot run and that's how I fixed the problem.