there are a lot of ways to keep a bevel angle constant. the one I use on scandi grinds is similar to what you see in the knifemaking videos of "greenpete's knife making" on youtube. Very easy way to file guide. Though i set mine up for draw filing.
Once you've filed enough to have the bevel set, you can use marker and freehand draw file. After draw filing I use 150 grit on a sanding stick (in my case, 1x1.5 ipe scraps, but a good piece of steel works), then 240 and 320. Then I do my heat treating and start back with anything down to 80 grint depending on how thick I left the edge.
Really, though, I'm sold on convex grinds and i do those with a slack belt and.... practice.