Plates with over 200 colonies were usually counted by dividing the plates into equal sectors (from 1/2 up to 1/8). After counting one sector, the count was multiplied with the total number of sectors to estimate whole plate CFU count.
But this way will lead to inaccurate count. The colony counting problem can be easily solved by using the "CountThings By Camera" mobile app. You only need to take a photo, click to identify, and the whole process can be automatically completed, and the accuracy can also be adjusted.