It looks like you want the buckets to be of even size. If for some reason you wanted to have more control over the bucket sizes/ranges (also useful to be able to lump outliers over a certain value into one "above X" bucket, you can do this, for example:
_your_search_ | fields some_field | eval some_field_group = case(some_field<50, "0-50ms", some_field<100, "50-100ms", some_field<150 , "100-150ms", some_field<200, "150-200ms", some_field<250, "200-250ms", some_field<500, "250-500ms", some_field<750, "500-750ms", some_field<1000, "750-1000ms", 1==1, ">1000ms" ) | chart count by some_field_group
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