Beginners seem to have a lot of trouble getting the flats off the sides and bottom of their stocks. Mabey it's from handling cheaper production guns or even new centerfire stuff. At any rate here is a layout tool, used by spar makers to lay out tapered timbers for making masts etc. The gauge here is of course small, but it can be any size. It will lay out lines on tapers or funny shaped wood at the correct ratio for eight siding. I know that there is a ratio for spacing the stops and the markers, but I can't remember it so here is the "manual" way to get the spacing.
Rip a piece of wood square 1-1/4" or 1-1/2" and clean the end grain. Cross mark the end from corner to corner then strike an arc from the corner to the center. Carry the arc to the flats and mark all of the flats. The stock,(second piece of wood) which carries the markers (pencils) should have about 2-1/2 inches between the points and the cutout should be 1/2 inch or so deep.
I made this one from a scrap, one inch thick by 2" wide. The width is important because it needs to carry a pencil that has been sharpened in a P sharpener , the taper is about 3/4" long and will not be supported. The length should be enough so that it won't get lost easily, 6 or 8 inches..
Set the gauge over the square stock you ripped and transfer the lines that were struck with the dividers up and onto the gauge. If you rock the gauge to the opposite angle the lines should line up. If that is true, droll the holes through, on center, with a 5/16 bit. The pencils should slide through with a little resistance.
Chop a simple slot into the sides of the pencil holes with a morticing chisel and fit a wedge into each one this will keep the pencils from falling out . By laying out parts of the stock and cutting down to eight sides, it's really hard to get a flat spot, this little gauge self adjusts to changing widths within the scope of it's size and should be handy for any rounding up work.