Just to add on to Dave's suggestion, I would definitely use a marker to help... it's pretty easy to lose track sometimes since you are going diagonal - my eyes start to blur from it.

I would also suggest counting several mid-body rows to get a more accurate average count.
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Chris Carille
Marist College and Mount Saint Mary College, NY
Department of Biology
Chris Carille Photography - carillephoto.com
Garden of Eden Exotics - edenexotics.weebly.com
http://nyexotics.blogspot.com/