I actually started on one yesterday, I did a good bit of research before starting to make cuts, from what I gathered it doesn't matter so much on the angles as long as they are all the same. The larger the angle from the axis of the pipe the shorter the collector will be but also will not flow as well. So it really depends on how much room you have to work with to make that compromise. Here are some links I found that were helpful.

http://teamhonda.userboard.net/t211-building-a-turbo-manifoldstep-by-step

http://www.toymods.org.au/forums/tech-conversions/11550-how-turbo-manifold-design-need-your-thoughts-2.html