Since you counted for 10 minutes, the instrument must have detected 2250 radioactive disintegrations. The Poisson distribution explains the advantages of counting your samples for a longer time. For example, the table below shows the confidence interval for 100 cpm counted for various times. When you count for longer times, the confidence interval will be narrower. The table below shows percent error as a function of the number of counts.
Percent error is defined as the width of the confidence interval divided by the number of counts.