RE: How to go about setting up a box?
Take a length of string. The longer the better. I use 20m (+-60ft) nylon chord used for clotheslines.
Fold it into 3 equal lengths. Tie a knot in the two folds and join the two loose ends together.
You know have a triangle with 3 equal sides. The inside angles are each 60degrees so it does not matter which side is which.
Place one corner at the pilot position and one side pointing to the centre flag.
Pull the other two sides taught. This will be the 60deg angle to the one corner flag.
Flip the triangle over (keeping the side that points to the centre flag). This is the 60deg pointing to the other corner flag.
We use an electronic range finder (used by golfers) to range the distance. 150m to centre flag and 300m to corner flags. I made up some 50mm (2") pvc pipes with plumbing fittings. 3 lengths gives me 4.5m with a cloth flag. The pvc poles are strapped to a fence "Y" dropper hammered into the ground.
String folds up small and can be taken along anywhere.