I've not used Metylan but suspect that the warning is entirely unnecessary.  I guess it may be a matter of the proportions.  In other words, if you get the proportions wrong you may not get the results you want.  It is ridiculous to suggest that you must make the full quantity and nothing less.
All you have to do (I feel sure) is to measure the contents carefully, either by weight or by volume and dissolve it according to the instructions and with the appropriate percentage of the water.  Keep the unused powder well sealed until you need it again.
With CMC (which may not be the same as Metylan) I find that keeping it in the fridge (carefully labelled!!) it will last for aeons.
DavidO

I've used it, and multiple small batches work just fine.  The instructions on the box are for use with wallpaper.  For PM, just make up a small batch at a time, like a cupful (or even less), but remember to keep it covered.  It's quite forgiving -- if it seems too thin, add a bit more; if it's too thick, thoroughly mix in extra water (just a little at a time).  I find the consistency of watery eggwhites the most useful.  Thick and gloppy is not good.

Nothing works as well as experimenting.  Trust me, if you make a really good mistake, you WILL remember it!  8-)

Sue