The suffix {-lI'} is to be used when there's an ongoing action, proceeding towards a known goal or stopping point.

So I wonder..

If there's an ongoing action, taking place during another action, or during a specified time period, then couldn't that ongoing action be "by definition" described by the {-lI'} ?

In other words, if we have a {-taHvIS} during which an ongoing action takes place, then couldn't this ongoing action *always* be expressed by the {-lI'} ?

With the "stopping point" of the {-lI'} being the end of the action/time period described by the {-taHvIS} ?

What if the while-clause doesn't have a known stopping point?

DIngtaHvIS tera', ghurtaH yIn.
While the Earth spins, life is ever-increasing.

You wouldn't use a -lI' for that. Sure, life might stop increasing when the Earth stops spinning, but life is not progressing toward the day the Earth stops spinning. -lI' doesn't just mean there is a known stopping point; it means you're making progress toward it.

What if the while-clause has a known stopping point that the main action doesn't share?

cholchuqtaHvIS mangghommey, mabomtaH.
While the armies approach each other, we keep singing.

We don't have to stop singing when the armies clash. We could continue to belt out our song joyously as we dismember our enemies. Our song isn't making progress toward the clash.

Don't look for a formula for this. Consider the grammar and choose the suffix called for by your meaning.

-- 
SuStel
http://trimboli.name