It seems to me that the mod doesn't kick in until after some drones are already sent out, so they go the maximum possible distance and the rest are then affected by the shorter range instigated when the mod kicks in. I take it this is not supposed to happen?

When there's nothing in range, the range will be at the range you have selected in the personal range setting.
For performance, the mod only checks for things to build every so often (update rate setting), so usually there will be some things in maximum range before the mod kicks in.

If I set a personal range to a smaller amount, will the mod increase it automatically until every task in the max range is completed, and then set it back to the shorter range?

Not sure exactly what you ask, but the easiest way to see how it works is to pick up a deconstruction planner.

The personal range is the maximum allowed. It'll never set it to a longer range than that. I usually set it to one roboport mk2, so I can have a lot of roboports without the range going crazy, and if there's some exceptional case like building while going afk I set it to unrestricted.

When everything within the search range is built, it's left at the maximum range. This makes it possible to build most things in a closest first manner without hogging performance too much, and still have a high maximum range.

However, if the personal range is less than or equal to the search range, I guess it could be left at a short range, so you wouldn't experience this. I'll have to test it out some time.

I'll have to play with it and see what results I get.

Thanks.