Phyton-27

[Ray]

A quart of water may weight 2 pounds, but the specific gravity of the Phyton-27 will be higher.

The simplest thing is to just use weight, but the addition should end up be the correct percentage of the total. For example, when I make up a 25% solution of fertilizer to make a liquid concentrate, I add 1 pound fertilizer to 3 pounds of RO water.

Your first post said 21%, now 26. Which is it? I would stick with 20% (1 part by weight added to 4 parts water by weight) or 25% (1 to 3, like my fertilizer example) for simplicity - which ever is closest to Phyton-27.