Why not lay a scrap board on the deck, projecting out a couple inches past the top nose, and then lay another scrap on the stair, touching nose to nose to nose. Overlap them at the top, set your bevel gauge on either one, and 'connect' (align?) the gauge to a line that passes through the top of the two boards to the bottom of the two boards? That's your miter angle, whatever it is (the number doesn't much matter). Cut the two scraps on that angle, put them back in place (on the deck and on the noses) and confirm that your angle (your cut) is correct and your have the miter, the look, you're after. As mentioned by someone earlier, the handrail piece will be parallel to the noses of the stair, so the scrap piece laying on the noses will be at the correct angle, and having the two scraps touch each other side by side lets you easily use your bevel gauge to set the required miter angle.
Don't let it bring you down,
It's only castles burning,
Just find someone who's turning,
And you will come around
Neil Young (with a little bit of emphasis added by me)
Board member, Gulf Coast Woodturners Association