Mitre saw angle above 45 degre

Hi Gang I really like the internet I feel closer to home than ever even if I m stuck in Quebec canada instead of the warm beach of LongBeach Ca.,futher more I sure glad Bob is on the net...thank you for that,for my projet I install a bay window and outside I redid all the coverage in a style of log house so far so good but when I finally got to the end ( the bay window) cutting in angle above 45 degree and in a slope it when I decided to find help I ve been trying but I just can get the perfect edge I got the angle but my saw doesn t cut at 67.5 to get the perfect cut can you help me please......

Jason