http://stellafane.org/tm/newt-web/newt-help-scope.html
That site seems to recomend that for visual use your 100% diameter should be half of the field stop that of an eyepiece that you intend to use. So for your 30mm it would be 15mm. The site also suggests half an inch of illumination would be a good amount. I have used their ray tracing to design my scope and I would say that it is very reliable but I have no experience with visual observation.