I figured out a good way to line up the initial file strokes accurately because it's a nightmare trying to keep the file edge from slipping across a freshly jointed (or flattened) saw plate. The trick? Clip a set of teeth you don't intend to use--and thus don't mind nicking with a file--so you can guide the initial strokes along the existing teeth in your sacrificial piece.
It was very tedious at first, then I got the feel for it and worked out how best to present the files and such to the plate, speaking of which, next pic:
Took the files you see back there and the little curtain rod holder with a notch cut out of one side to get the teeth angled properly, easy starting -30 degree rake on the first two pairs of teeth, dropped to about -20 degrees for the next two, -10 or so for the next two. The way the tooth leans back some as it contacts the wood makes the front glide into a cut like buttah' wif one-a dems fuckin' stabby things whats ya done heated ups!
Then I keep the saw file held so the face pointing towards the heel/handle end is vertical which means the rest of the teeth are presented aggressively for almost violently fast cuts, they're neat looking too, I think, though you can't see the slight bend every other tooth has to the right or left side of the plate, doing that means you remove more material and leave a wider kerf than the upper part of the saw plate which keeps the saw from dragging and binding in the cut. I used the notch in the little curtain rod holder to kink the ones you see with a shiny face so they bend away slightly, while the ones between them bend slightly towards the side facing the camera:
Just needed a couple of strokes to get as far down into that block of wood as it went, didn't need to force it, fiddle with making little pre-cut guide marks by dragging the saw backwards first, or any number of suggested tricks you can find on any number of "how to saw perfect dovetails" or whatnot tutorials over on youtube. Myself, I would suggest you rest the front teeth on the upper edge closest to you and straighten your arm through the air above the wood you're trying to cut for the starting kerf, then just press the teeth back through with that slight upwards/forwards motion like you had a line on the sides and ends of the board and wanted to trace both lines with the saw. You'll wind up crosseyed trying it directly though, just make like Perseus and use the side of the saw as a mirror, if the wood looks like it continues on a straight line through the reflection, congratulations, your cut is dead straight to the back face! You can kinda see the mirror effect in this picture actually:
Final shot with the relevant tools, all of which are either totally made by me or in the case of the files/hacksaw blades/screws and bolts, assembled into relevant spots of pieces made by me, though the file handles and saw vise aren't pretty, as they were put together to fill a need as I needed it, unlike the saw handles which were more thoughtfully formed and fitted for a comfy and useful grip.