Using a chrony once will only give you what happened on that day. If you just want velocity for factory ammo, it should be cheaper to find someone locally that would set theirs' up for you and show you how to use it. If you are handloading, one time will not really do you any good, as you will have it available for only that one day. If you are just going to make up one load and be satisfied with whatever it gives you, one of the locals is still your best bet.
I am trying to envision a scenario where a one-time use would be beneficial enough to justify $30-40 expense, but it's not coming to me. It can be very enlightening to use a chrony for working up a load, but that doesn't sound like that is what you are doing.