First Guess for nth Root

I want to add an implementation of the PrimeSwing algorithm for computing factorials to my math library, and it requires both prime sieve and square root functions. So, I'm working on add nth root support first. I found this article on Wikipedia on the topic.

The first step is to "take a guess". I'd like suggestions. Presently (on paper) I'm saying that if n is greater than the orders of magnitude in A, then pick a single-digit number. Otherwise, divide A by 10 * (n - 1) and then further divide it by n * highest order digit of A. Ehh. It gets roughly close. I'm sure there's a better solution.