If in a multi or mystery geocache not all variables for the solution of the final coordinates can or could be determined, it is sometimes a good idea to simply look at all the open options, maybe even visualized on a map. In this way, some options can often be excluded (Of course, the possibilities should be very manageable. Nobody benefits from seeing 1000 possible coordinates).
The Variable Coordinate tool can be used as a stand-alone tool. But it shows its real power in combination with the formula solver. If a formula could already be filled with some values there, you can copy the partially filled formula and paste it into the Variable Coordinate tool. The missing values can be interpolated here.