That works! A variation on the theme is to draw the opening on part and if possible scribe the lines so as to have something if working the part wears off the mark. If the part is flat enough I’ll often follow the scribing with a light pass or two of the fabled #11 blade.
Then as Rooster said, drill, file, carve. I’ll often carve the corner down close then file to finish. On really good days carving alone gets me there!
Hmm, the interesting random phrases which arise from the hobby:
“tiny angle shim mounts”
“file to finish”
“carving alone gets me there”
“working the part wears off the mark”
Ya know, someone ought to model a locomotive supply dealer equivalent of a ship chandler and have a sign up for “Sale, 10% off tiny angle shim mounts” Which hopefully will give rise to, “Wait a minute, what’s 10% off, the money or the angle?” (http://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-wink.gif)