I am an expert moving a large household numerous times.
I always provide lunch, tap water out of cups with ice and depending on the performance I give them tips. Usually I give $20 each per day. They always are really really happy about the tip. I guarantee that they do not see a tip on a regular basis. I have had it where a company hired goons through U-haul to unload, and these guys were a mess. No tip.
So, my input is to not overdo the tip and rather get them lunch. They'll appreciate the gesture. $40 is way too much for a cross town 1 day move IMHO.
George