Thank you!Yes on the 4x question.
Yes, you can add it based on weight if you have an idea how much moisture is in each product (which can be hard to know.
The mass ratio of formic acid (2 x molecular weight) to calcium hydroxide (1 x molecular weight) is 2 x 46 to 1 x 74 = 92:74 = 1:0.80
If the certification for formic acid states that the concentration is 85%, should I take this into account when calculating...?