Determining lenght to double side flush rivet
You can determine the correct length by setting up a test strip first and experimenting with your squeezer. Also, you must be careful to keep pressure on the squeezer so that the rivet remains tighly seated in the "top" countersink while you are filling the "bottom" countersink.
If you are back riveting if possible clamp the parts to the backing bar. This will insure that the rivet doesn't back out of the countersink.
On some occassions, you may find that none of the rivets you have on hand are the correct length. In that case you need to have a rivet cutting tool so you can cut a rivet to the exact length you need. If you are building an RV-12 which makes use mostly of POP rivets, the investment in a rivet cutting tool is probably not worth the money.
You can make a simple rivet cutting tool with a pair of pliers. You open them slightly, then in your drill press, drill a #40 hole on one side and a #30 on the other.
Then when you insert the rivet and close the pliers, the rivet is cut cleanly. I once had a pair of pliers that I modified to do this. Note: not all pliers are tight enough to give you a clean shear cut.
"If" you have a steady hand, you can cut a rivet to a shorter length with a dremmel cut off wheel. Wear safety glasses. This is only practical if you have but a few rivets to shorten.
Tom Hunter