Let's say you have a "form" of the estimating equation that you want to use. In your case y = a Ln(x) +c and the problem is to determine values for a and c. In your case Excel has told you that a = 29.745 and c = -101.84.
You could calculate these values yourself using matrices.
We have the matrix equation:
Y = X*A where:
Y is a column matrix containing all of your coverage values
A is a column matrix with just two elements being a and c
X contains two columns one full of 1's and the other containing Ln of your TFNs
Now we need to solve this for the matrix A so that we get a and c.
Y = XA
is what we call a pseudo inverse of X.
I am going to guess that curve fitting in Excel may be the easier option for you?