How to calculate error in successive over relaxation for PDE?

I am trying to solve the Poisson equation numerically using the FDM method in C++. But I have a little confusion with the iterative process. I understand that the number of iterations should go until the solution converges, but how to calculate if the error is greater/less than the tolerance level? Here is a small piece of code I tried in C++ but it is flawed. I checked some other codes posted online and some has calculated the average of the residual values in a matrix and checked accordingly. I’d appreciate it if anyone could help me with the concept.

void calculate_voltage(){
    voltage_initialization();   //creating a matrix V and initilizing with Dirchilet's boundary condition
    double tolerance = pow(10,-5);
    bool done = true;
    int itr = 0;
    double pi = 3.14;
    double t = cos(pi/nx) + cos(pi/ny);
    double omega = (8 - sqrt(64 - 16*pow(t,2)))/(pow(t,2));  //relaxation parameter
    
    while(done == true){
        itr ++;
        for(int i = 1;i<nx-1;i++){
            for(int j = 1;j<ny-1;j++)
            {
                double vv = (V[i-1][j] + V[i+1][j] + V[i][j-1] + V[i][j+1] + step_size_ * source[i][j])/4.0; 
                double R = vv - V[i][j];     //residual for SOR
                if(abs(R) <= tolerance){done = false;}        //to check if the correction converges or not
                V[i][j] = V[i][j] + omega* R;           //new V
            }
     
            }
        }
    }