My suggestion is that there should not be an audio cue, because I'm looking at my screen and I don't need a boring sound, because I have a wide enough field of view to focus on the chat window and see the countdown appear. Sheeps are amazing, I know. And if you aren't looking at your screen but are not AFK, you don't need more than half a second to look at your screen.
My suggestion to RTFM is to make you less anxious about not getting that mex beside your startpos because a teammate that wasn't playing with his
* was faster than you to build it.
However, you should be anxious about plopping the same fac as your neighbour because you didn't announce your fac. What is the last game you played where you announced your fac or actually discussed strategy with the team? Because "being fairly sure of the plan" alone, and having discussed a plan with your team are 2 different things.
If you are disappointed that your idea didn't get the positive feedback you hoped for, it may have to do with the idea itself or your ability to explain it. Seeing the effort you put to explain your idea, and the lack of interest we can see through posts in this thread I'd say your idea isn't compelling.
If you had proposed something more useful, like a selection box displayed during pre-game where you can pick a factory and that add a factory label near your startpos, I'm sure it would have got better feedback, because that would help team cooperation rather than help a distracted individual to shave a few seconds off...