I finally managed to set a bevel using 1200 grit sandpaper, thanks to jelajemi for recommending that, but now I can't get it to shave arm hair without touching the skin.

I've tried starting from 25 and following this pattern Pyramid honing guide - Straight Razor Place Library
And then following the aggressive one from here Honing - Alternative approaches - Straight Razor Place Library
And then following the conservative method twice.