A taylor series expansion of a function V(x,y,z) for a 'step' of size a in x gives

                    V(a,0,0) = V(0,0,0) + a ¶ V/¶ x|_o + 1/2 a² ¶²V/¶ x²|_o+ terms in a³ and higher .

In slightly different notation we could write

                    V(a,0,0) = V_o + aV_x + 1/2 a² V_xx + ...

                    V(-a,0,0) = V_o - aV_x + 1/2 a² V_xx + ... .

Then we could solve for V_xx and find

                    V_xx = ( V(a,0,0)+V(-a,0,0) - 2V_o )/a² .

Likewise
                    V(0,b,0) = V_o + bV_y + 1/2 b² V_yy + ... and

                    V_yy = ( V(0,b,0)+V(0,-b,0) - 2Vo )/b² .

Laplace's equation is V_xx + V_yy+ +V_zz = 0.   For a step size of ± a in the x-direction, a step of ± b in the y-direction and ± c in the z-direction we may write down laplace's equation and solve it for Vo:

            2 Vo (1/a² + 1/b² + 1/c²) = ( V(a,0,0)+V(-a,0,0) )/a² + ( V(0,b,0)+V-,-b,0) )/b² + ( V(0,0,c)+V(0,0,-c) )/c² ,
or
           Vo = C_x (V(a,0,0)+V(-a,0,0)) + C_y (V(0,b,0)+V-,-b,0)) + C_z (V(0,0,c)+V(0,0,-c)) ,

where C_x = 1/(2a²)/( 1/a² + 1/b² + 1/c²) = 1/2 b² c² /(a²b²+ b²c²+ c²a² )etc.

If a = b = c (same step sizes in x, y, and z) we find Vo is the average of all the cells around it:

Vo = 1/6 ( V(a,0,0) + V(-a,0,0)) + V(0,b,0) + V(0,-b,0) + V(0,0,c) + V(0,0,-c) )

(In any case, Vo is the average of all the cells around it, but it's a weighted average over the different step sizes if a¹ b, or b¹ c, or a¹ c)

Where there is no z-dependence, V(0,0,c) = V(0,0-c) = Vo. Then if a = b,

                    Vo = 1/4 ( V(a,0,0) + V(-a,0,0)) + V(0,b,0) + V(0,-b,0)) .

The plan is to repetitively solve for Vo in each cell by using the iterate feature in Excel under Tools/Options/Calculation. By setting 1000 iterations, you get that number each time you press F9, in manual calculation mode.