A novel idea but not worth the trouble from either perspective. You'll learn shortcuts (train 5646 troops instead of 5980) to save yourself the inconvenience of clicking on multiple different boosts. There are several boosts/manipulations that would make this impractical, sorry.

Carry on and have fun!

I get the impression you like to work in round figures .

However here is another alternative.

Use the slide bar till it hits the level of the boost you have, or what you want to use. You can always train more troops later on.

Though more troublesome a variation on this is:

You wait  a required amount of time at the beginning or end of training to accommodate needs before engaging the boost.

I hate to waste time boosts too, especially on tasks, where its more problematic to work things out.The numbers vary so much and are not exact.

The same also applies to research and buildings, and its even more difficut when you get the 10%extra time  bonus or use the other kind of boosts as well.