Minecraft's FiniteWater mod: what is counted as “low in mass”?

I've recently installed DJoslin's FiniteWater 5.6 mod but am having a little bit of difficulty with how it works. The site says “Any water disconnected from the ocean and low in mass will be finite and react to changes in volume.” — what is classed as “low in mass”?

My plan was to block off a section of this river and drain it. I've worked out how the pipes work and the pump needs to be at the end of the pipe section, and it sucks water up as expected but the water level doesn't go down.

Do the same thing with a nearby smaller pool and it works fine, with the water being emptied out.

And finally, with a bigger (but shallower) pool than before, still no go.

I can plonk a “grate” down in a trench I've dug for example 2 and the water floods back in (annoyingly, regardless if there is a Redstone input to it or not) as finite water. Take out the grate and let water come in, that's all infinite water.

What is the largest body of water that is “finite” — or do you just need more pumps?

I asked on the mod creators' forum and got the answer of, by default, the limit is 1024. If you open up your Minecraft folder, go to .minecraft\mods\DJoslin, there is a finite_liquid.txt that can be opened (with, say, notepad) and change the OceanToLakeLimit ; 1024 number to something higher, say, 2048. I've tried 4096 which has worked on my laptop without issues and there is no apparent reason why higher numbers wouldn't work.

I do notice a larger area can be pumped out now.