Here are some things I do:
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Set all factories to auto assist by default (Settings/Unit Behaviour etc..). This makes constructors assist their factory when they leave so they are at least doing something.
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I have unit command visible set to the level that shows all the commands all the time. It is easier to spot idle units with this setting.
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I press the idle worker button somewhat frequently. I have it bound to Z and I think that is the default keybind.
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For (non-unit) construction in my base I (ideally) queue a guard order on my for after the construction is complete.
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I can queue the expansion for entire sides of maps all at the same time. To do this rapidly I use the area mex command then command insert any turrets or solars that I might want. I think these are the only types of long complicated queues that I will set up.
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I probably make too many constructors to counteract my inefficient use of them. Constructors are the cheapest type of economy and the most versatile. They can reclaim and repair. Having a few too many is not that bad. An idle worker is only really bad if your metal storage is filling.
My factory order queue is almost always a single move command so I wouldn't recommend issuing large repair or reclaim orders. In any case, my constructors would never reach those orders since I use auto assist.
Another thing about constructors is that they gain value by being further than your factory. If you think about it a constructor at your factory costs, say, 140 metal, energy and build time, whereas, a constructor on the other side of the map has a large time cost on top. If you take my advice to err on the side of overbuilding constructors and find yourself with too many just try sending the excess constructors further from your base. You'll likely find something to do with them.