In my opinion, when dealing with "overtaking" a convoy, you need to be able to use the UZO correctly. As Von Due stated you need to know a general course of the convoy and speed.
You can't use nearest visual target as this usually is an escort, and the move around too much, and change speed often.
Pick a target as "reference" in the convoy, use the range lines on the UZO, and determine the range. This does not have to be entirely accurate as you want a general course. Mark it on your map and do this every so often.
Mast height X 1000 / (mils X 10) = range
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