Just a couple more tips. At your end point, don't fill the tank completely full. Pump in a gallon and see if you get a good receipt. If so, the clock is stopped and you have plenty of time to add more gas. However, if the dated business receipt is lacking critical info, you have more room in the tank to go get another receipt across the street at a different gas station. I admit to dumping $0.10 of gas on the ground to get a second receipt once. And very small gas station credit card purchases often trigger fraud alerts that sometimes require calling the CC company to get removed.
First step if you get a bad receipt is to go inside and ask for a receipt for pump # what ever you're on. (remember to check the pump # before goign in, it makes it a lot easier if you know). If that receipt is still bad, ask if they have an ATM and check your balance, getting a printed receipt. All this takes some time, but is faster than going to a second unknown station.
If you're still needing a good receipt, time to hit the road looking for another place. If you know you will be very close on time limits, you should have an alternative location already planned.
I like the alternate method myself. I usually get my wife or neighbor to document the odo the night before and depending on how late I get in, the morning after. Ideally you don't travel too far between your end receipt and your end odo witness, but just add a note if you did so they can see how that fits.
Another tip - If you ask a Police or Fire/public officer to sign your witness form, don't call it a witness form. Some won't want to "witness" anything, as it seems it might put them in a liability situation. Witness means something more to them than to us. Call it "document". As in, "will you document my odometer for me, I'm doing an Iron Butt Association ride and need someone else to document my ending/starting odo".