There is a wizard that does just that. You touch off 3 points on a circumference of a circle (or 4 if you like) and it automatically calculates the centre. You probably knew it, but just in case
And while on the topic I will add another method I use to centre a chuck on the mill's table. Hold a round bar in the mill, lower the head and chuck the other end of the bar in the chuck. Now it's perfectly centred, zero the DRO's and use clamp the chuck to the table. Fast and easy.