A tip on how to set up for countersinking
When I started this venture, I was always puzzled when trying to figure out how to make a countersink the correct depth.
First of all, test on a scrap piece! That is the most important step! Hardware store aluminum angle works fine for this.
Also, always back the piece up with something if the piece is too thin to stop the countersink from wandering. If you don't, it will meander all over the place, making the hole to big and oddly shaped. The little nub on the bottom of the countersink tool should be fully in some material. You can use aluminum/wood/thick plastic or whatever drilled to the hole size. This will be important when you do the counter-sinking on the wing spar to attach the fuel tanks to the main spar flange.
A friend showed me that the easiest way to set the depth of the countersink is to measure the width of the hole. I'm talking about the top of the surface where it is widest from counter sinking it.
I do this using a dial caliper (micrometer) to check it. It should be the same width as the widest part of the flush rivet that you want to put in there. You can get it very close to the correct depth this way. Then test it by sticking in your rivet to fine tune the depth. After it's good to go on your scrap piece, it will be good to go on your plane.
It works for me. Your mileage may vary as they say. I hope this helps you for future holes... Happy building!