C = lerp(A, B, 12*dt)
The 'lerp' is of no importance. See it as a range.
C is in the range (A to B), where B = 100 and A = 0%
Say A = 2 and B = 6. Then the range is 4.
Then 100% = 4
Normalised 1% = 4 / 100, or 1 step = 4 / 100
Say C = 2 ... how many normalised steps to make that 2 ? Well 2 / (4 / 100) = 50 ... To 100 that is 50 %
The lerpfactor goes from zero to 1.
So in .. 2 = (lerp,4,6,x) ... x = 0.5
Generalised.
lerpfactor in % for C between A and B = C / (B-A/100)
That also gives negatives. When A is bigger then B. Up to you how you wrap you mind about that.
For me that just means that the range has a negative growth.