Sorry, that doesn't sound like a very sure way to achieve toe-in.
Fold the card double and trap it at the back of the brake block while holding the brake lever on. Loosen the bolt holding the block, allow the washers etc. to settle in the new position then gently tighten, while holding the block against the rim with the lever. Once it's nipped up enough to stop the block from moving, release the brake and finish tightening the bolt while bracing the block with your fingers to prevent it from rotating. The brake block should now be toed out by two card thicknesses at the rear, so that braking action drags it flat against the rim.
Rim brake squeal is caused when the wheel drags the brake blocks forward, bending the caliper arms and causing the blocks to move out of parallel with the rim. They lose grip, let go and bounce back, setting up juddering or vibration as the cycle repeats. By toeing in the blocks you "pre-stress" them so that the wheel pulls them parallel with the rim.