[math-fun] Prob. that 2 numbers relatively prime, error term?

A018805(n) is no. of pairs (x,y) in [1..n]X[1..n] with gcd(x,y)=1. It is ~ 6*n^2/Pi^2 + C*n*log n, I think. What is C, and how can one find it using Mathematica or some other program? I ask because there are several similar sums I am interested in. Is there a reference? Background: A331755 has an explicit formula (involving A018805) and is ~ 9*n^4/(8*Pi^2) + O(n^3 log n). A331763 is a mystery that I would love to solve, and seems to be ~ constant*n^4/Pi^2. Maybe knowing the second terms in the asymptotic expansions would help with the curve-fitting.