Try determining the radius of a circle which would enclose your plants roots, then make a cylinder one inch high from this. Perhaps you have a container laying around with a bottom close to the radius of your plants root system, and at least one inch high that will hold water? Then use your two litre, dump into the container until it reaches at least one inch in depth!
Now this would assume even absorption of water throughout the ground, so you will either need to move the bottle between fills, or fill more often... Just an idea, maybe someone else has a better one?

I just go by look and feel, but if you want 1" of water on 18" dia . root zone it will take approx. 1.1 gal. of water to do the job. 1 gal= 231 cu. inches. .7854 is the ratio between a square and a circle.

Larry

HUH? I usually just poke a finger in the ground to see if it's wet enough. Cuz I'm math challenged.