Auto/preplan cuts? (using bottles & airpressure)

[CheshireFool]

Sorry if this is discussed elsewhere,
Mostly I'm fuzzy on the language, so search results get me nowhere.

If you take a coke bottle, put two straws in it, and then seal the top with modeling clay, you can pour water into one of the straws and slowly fill the bottle with water. Until that is, the water level plugs the second straw and the air pressure in the bottle stops any more water dripping in.

If you know your amount of wash, would it be acceptable practice to set up a 50ml bottle (for 20L wash) with such a configuration to automatically take off the first 50ml?

Or use a series of bottles if you want to collect the heads or tails in less than 1L increments?