First off, no there is no system imo.

When I have no reads on someone, I play against them as if they were a "standard" player for that level.

You can kind of think of the value of hands vs certain kinds of boards, i.e. top pair isn't great on a 4 flush but this isn't absoute. I used to say to myself "I'll never stack off with a jack high flush again!" I was very wrong to think this way. Think about your opponents possible holdings given the board and the action, that's really the best you can do.