I have 3 values that depend on each other. For example:
pace = time/distance (note:
speed inverted). The user can edit any of them and one of the values should be updated based on the two others.
The user might prefer to say
run 10km in 1 hour and pace would be updated. Or he would like to say
run 1 hour at 6:00 min / km pace. Or he could say
run 10 km at 6:00 min /km pace. All these three options make sense in different situations and the user preference can change between different data entries.
Now, when the user enters two values the first time (say pace and distance), it is easy to update third value (time). However, when he later on edits third value (time), it is non-obvious, which one of the two values (pace or distance) should be updated and without good UI it can create a confusing situation for a user.
What are good UI designs for situations like this? How to make it understandable for the user, which one of the values will be updated? Any nice designs that allow user intuitively to select the dependant value without putting too much though on it. Any good examples on the web? Or pointers to books where this is discussed?