At least for my setup the point is to prevent the gnomes from trying to fill up a stockpile immediately as one item is removed from the pile, but to wait until the stockpile item count is under the threshold, and then fill it up to 100% again. I.e. to delay the start of stockpiling to encourage efficient wheelbarrow/etc use and allow gnomes to focus on fewer tasks at a time, but NOT to limit the size of the stockpile at all. So it's about the amount of items, not free tiles / containers. And this would be a permanent tweak to maximize stockpiling efficiency, not a temporary thing to clear space or whatever.
100 logs in a stockpile (100 logs capacity), threshold set at 50. A gnome starts cutting the logs into planks, one by one... Instead of gnomes starting to run around piling logs one at a time to the stockpile, nothing happens until the stockpile hits 50 logs. At this point idle gnomes rush in to fill the stockpile using wheelbarrows, filling the pile up to 100 logs.
I don't know how Gnomoria handles this, but I can see a problem if a gnome manages to fill the stockpile to e.g. 99 logs, and another gnome takes out a couple logs, leaving the stockpile to 97 -> Should the stockpilers now only fill it up to 99 and wait for the pile to hit threshold again, or go straight for 100? I'd say 99, otherwise we again have gnomes fetching one or two logs at a time... Not sure though, just thinking aloud.
And I would be very happy with a general setting. And as mentioned before, this should apply for all stockpiling regardless of source, not just push/pull-piles.