You can generally boil two cups of water with a half-cube of Esbit. If you have problems, then it will require slightly more. If you are very good with it, it may require slightly less, but you aren't that good with it. For example, your windscreen, your lid, your placement out of the wind as much as possible, a stable ground underneath it that won't act as a heat sink. Your Esbit burner must be good. You have to know which way to orient the Esbit half-cube (flat, on edge, or vertical), etc.
If you are afraid that you won't get a full boil on two cups of water, then you can reduce slightly the amount of water. Or, if you have your cold water in a sealed flat container, you can put it under your shirt for a while to let it warm up to body temperature. That will help out just a tiny bit.
When I pack up for a trip, I figure (worst case) that I will have a complete stove failure, so I understand what I will do with cold food and beverages and I plan accordingly. Now, 99% of the time, I have no such failure. I just like to be prepared for what can happen. Probably 50% of the food that I carry requires no cooking or boiling.