Its a bit of a bugger to explain. I take it you know how to do a hard arris with a rule?
The side your doing lay on as normal, when you get to the angle hold the rule up and work to that, slide the rule off your work and return it lay on a trowels width of gear,working to the rule on the opposite side, slide the rule off again. you should have now a sharp angle.clean the rule and hold it up on the side with the trowels width of gear dash down the length of the rule, slide it off again and you will have a sharp angle where one side is dashed and a band of stuff down the other side, scrape the gear off the return and cut it into the angle when its dashed and it should protrude slightly allowing you to do the other side ,when you come to it just wet the angle and render to it. Failing that use beads