If you are making a bivy bag I suggest you consider incorporating a pointy top in its design. Here's an example with a green bivy/tent that hangs under a shaped tarp (fly).
The added pointy top, in this example, added about 1 ounce to the bivy's overall weight of about 6 ounces and required little, if any, more work than making a more traditional bag shaped bivy. The 45-48" height of the bivy allows me to get up on my knees when changing clothes and to hang things (e.g. light, wet socks, etc.) from the peak of the bivy.
With a pointy top the "feel" of the bivy is more like a tent than a bivy.