Having done similar things, I agree. 1) Make sure that the UI fits with the order they have data on the bits of paper. 2) Let them data in any order they like,

Having done similar things, I agree. 1) Make sure that the UI fits with the order they have data on the bits of paper. 2) Let them data in any order they like, and give them a big "Validate" button they can press to check whether what they've entered is all coherent. (Don't check as they go along, or the validation markers will confuse and distract them.)