Average consumption rates will be dependent heavily on things like light intensity/duration, CO2, plant species type and plant density.
10-30ppm seems fine for most folks for K+, you can go higher without any ill effects on most any species, I'm always curious to test higher levels on speculated "susceptible" plants. Never found any yet.
Here's Erik's tank with over 100+ppm:
Another with 30ppm:
It's very easy to hit such a good sized target with some water changes and dosing thereafter. 10+/- ppm is very easy to accomplish. Hanna makes a decent colorimeter for measuring K+. Lamotte and other test are so so at best due to low precision over the desired range of K+ testing. These can be reduced and diluted to improve the precision, but this adds a few extra steps.
My own tanks have never exhibited any sign of Ca or K+ issues and each time this was speculated, I checked the plants.
I had large stands of Rotala "green" in my 180 gallon for about 6 months, I chose another plant, it grew like mad. Dosing was very rich in K+, about 30ppm initially and then another 20ppm during the week, so about 50ppm added a week.
With some build up, uptake etc this means my K+ sits around 50-60ppm.
Where's my stunted tip growth?
If that is really the real reason in and of itself, I'm game to explain how come I add it, , give specific data points, and post the pictures of the resultants, and no stunted tips.
How is this? Magic?
I no think so........
I'd not worry much about nutrients being too high and simply dose and be done with it, then focus more on CO2, current, and light intensity.
They are a lot more effective for management. Folks focus too much on these slight differences in ppm's or nutrients and think it produces dramatic results.
Rarely is that the case. Light and CO2? Very much so.
I add about 45ppm a week of NO3, 10-15ppm of PO4, 2ppm or so of Fe as proxy for all traces. I also add another few ppm's of Ca and Mg, and K+ as GH booster after water changes.
Nothing special really.
You can tweak to suit and likely should.
If you do get stunted tip growth from adding more K+, I'd look at the CO2 again. Current and adjust, if that does not work, try less light intensity since that will reduce the CO2 stress and you should be able to dose more K+ without the same issues occurring, as well manage everything easier and leaner if you think that is somehow a key.
Plants like nutrients, CO2 etc, I add them, pretty simple stuff really.
I use light to limit/manage growth from there.