The conversion between flow at one depression and another is just simple math.

The change is the square root of the new pressure divided by the old pressure. Let:
C = change
O = original test pressure
N = new test pressure
C = (N ÷ O)^.5
Example: compare 28" to 25".
28 ÷ 25 = 1.12
1.12^.5 = 1.058
28" reads 5.8% higher than 25", so multiply your 25" reading by 1.058.

Remember, this is what a steady-state test instrument will show, not an actual head!