but then how do you know what the percentage is based on your opponent having random unknown cards before seeing the flop?
IF you are referring to pokerstove, you select their range of hands.. so it isn't unknown. If the hand IS unknown.. ie random distribution, it enumerates all the possible flop/turn/river combinations and sees how often your hand does against each possible hand they have (if its a random distribution, they can have any two cards from AA to 23os) and then gives you that figure as a percentage, effectively telling you how often you will win. if you want to know how to figure it out, you can use combinatorics quite easily to do it.. although its time consuming
If you are referring to the fact that sometimes your outs mite not be live (ie discarded from other players hands) then naturally this isn't known.. sometimes the deck will be light of cards that make ur hand, and sometimes it will be loaded with cards that make your hand.... this evens itself out over time, so you just go with what you know, which is your cards and the cards on the flop
If you are referring to something else, I cant figure it out from your post