Not that the question was directed at me, but, my name is James so maybe I have some insight ;)
I did not make my bivy but rather bought it and went with the side-zip option. It's an MLD superlight for reference. The side zip comes down to about the waist or mid-thigh on me and there is one monumental advantage for me; no shimmying to get into and out of my bag and bivy. I can sit on my pad and swivel on my bottom, slide my legs into the bag/bivy, lie (lay, I don't know) down and zip up and be done. That is why I really went with a side-zip.
P.S. we'll see if the next response is from a James or Jamie, I'd accept Jim as well.
[edited: forgot to say that I'm very impressed with your bivy, both weight and price. way to go!]