Still waiting for a rule citation.
FAQ, page 9, Measuring Range:
Players may only measure range and/or use the range ruler to determine whether a ship is inside or outside of a firing arc at the following times:
...
• After declaring the intended target of a target lock action, the active player may measure range to the intended target, and only to the intended target.
If you're using the target lock action to measure range to another target, then you're breaking the rules. There is a certain amount of unavoidable incidental information; taking actions which are specifically aimed at creating "incidental" information is intentionally measuring to a ship other than the intended target.
And? I can still measure to a ship that's way out of range for the target lock. So thanks for telling us we have to point the ruler to the ship we declared, which we knew.
The whole point of using this "thing" is to measure for stuff in between anyway.