Stop donut Chart resizing during drilldown

Hi all, 

Very frustrating, but hopefully easy problem to solve. I have a donut chart that has 3 levels to it. When I use the drill down, depending on the size of the labels in the legend, the donut increases or decreases in size.

The problem is that I have a card in the middle of the donut that states the total value of the chart, and this can overlap when the chart resizes.

Is there anyway I can stop this?

Thanks

JAmes