In order to compute the angle change, you need to know the front-centre. I'm confused as to what are your frame dims, but it should be around 600 mm. Dropping the BB 8 mm would then slacken the angles by 0.8 deg. It's significant but not excessive IMO.
BTW, I'm not sure how you can significantly increase clearance. Lengthening the stays will help, but there is a limit to which you can indent the stays (they likely already have a round-oval-round profile?)
Last edited by tuz; 02-04-13 at 08:33 AM.