Instead of using lerp like that, use it the intended way:
Lerp(min_scale, max_scale, percentage/100)
Where min scale is the minimal scale, max scale is the max scale, and you make percentage varies from 0 to 100 to vary from min scale to max scale. (For exemple when you are zooming in, set percentage to clamp(percentage+100*dt, 0, 100) will zoom from min to max in 1 second)