See e.g.
The same applies for any polynomial approximation. Thus, whether you use the log or not does not really matter. At first order the two specifications are even equivalent.
Regarding percentage deviations: if your mean Ass is 1, then you are getting percentage deviations as the log would imply a Jacobian transformation with 1/1=1. Put differently, if A is above the mean of 1 by 0.01 units, this is also 1 percent.