Reseal and use for next change. Almost embarrassed to admit this, but if you use the density specs provided by the mfg (compared to water) you can use a digital kitchen scale to add the exact amount to the old bottle to meet your refill specs. Hint: an empty 5 quart jug weighs about 250g.
This does involve surreptitiously removing and returning the kitchen scale to its rightful place though...
This does involve surreptitiously removing and returning the kitchen scale to its rightful place though...