I may have missed something, but I just had a guest book a space for 2.5 hours and include an hourly add-on for 1 hour. I know this is a new service but that is not how hourly add-ons should work.
I have now had to explain to the guest about paying more for the item (lighting package), and they are arguing that they technically don’t need it for the whole booking because they won’t be shooting for the whole time. That’s not how equipment rental works!!! But I can’t win this argument and not piss-off the guest. Boo! 
A “Guest-Determined” policy guarantees almost every guest will try to lower their bill by selecting fewer hours than the booking, meaning hosts will have to reach out and explain “that’s not how it works”, then modify almost every booking, which will result in constant fights with guests.
By default, ALL hourly add-ons should be set to the hourly length of the booking, otherwise the tool will cause more problems than the convenience it provides.
It’s much easier and more satisfactory to the guests for a host to discount a higher rate than to ask a guest to pay more than they expected.
If you’re not convinced, maybe take a poll of Peerspace hosts and ask us if we’d prefer hourly add-ons were:
a) automatically the same length as the booking (determined by host)
b) independent of the length of the booking (determined by guest)