Boom Vang Rule of Thumb

[Bluenose]

I am having an old Shield One Design sailboat modified to create a modern Knockabout type sloop and I am looking for a way to maintain a boom vang. The current Shields boom vang is quite shallow at about 18 degrees, plus or minus depending on the source, as measured from the boom horizontal.

You can see the shallow angle while sailing in this picture.



And here is class sail plan.



Here is my concept with the cuddy cabin. Obviously I have reduced the space for the vang (please ignore my vang mockup since it is just a place holder right now).



I have research various other class boats and some have quite shallow vang angle. My question is, is there a limit to this or a rule of thumb. Or better a set of design guidelines concerning minimum angle, distance from the mast etc. Any help would be greatly appreciated.

Bill
Lopez Island, WA