Is it natural to say "drink it up to this level"?

Question

I want my son to drink warm water in the early morning so that he can easily empty his bowel before going to school because it's not nice if he has to do it at school.

But, he doesn't like it very much. But I still push him to drink some.

Look at the picture above, I want him drink 80% of the water in the glass. So, I point my finger to a point (as shown in the picture) and say "try to drink up to this level".

I am not sure that is natural because "level" doesn't sound natural.

I remember someone say "I drank this much" when he show his fingers like this

Answer 1

The "up" in "up to" is referring to the direction that things (i.e. in this case, the water level) are changing to reach that point. When you drink from a glass, the water level does not go up, so saying "up to" in this situation does not make sense.

However, when you drink from a glass, the water level goes down, so you can say "drink down to this level" instead.

Some other examples:

• "Fill the glass up to this level" (filling makes the level go up, so "up to")
• "Drain the bathtub down to this level" (draining a tub makes the water level go down, so "down to")
• "Boil the soup until it has reduced down to half its volume"
• "Inflate the balloon up to this size"

Answer 2

Edit

To follow the OP's preferred method, as shown in the diagram, we could say

try to drink [down] to [here]

try to drink [down] to this level

(My earlier answer didn't go according to the diagram and was an alternative way.)

Earlier Answer

We could say the following while pointing at the desired level:

Try to finish (or 'drink') the amount filled to this level.

Fill to this level and try to finish it.