Takea 3/8" X 36" wooden dowel and wrap masking tape around one end until it fits the bore snugly, but not real tight. You want the tape tight enough in the bore to engage the rifling, but not so tight that you can't movethe dowel up and down in the bore fairly easily.

Insert the dowel fully into the bore, then back it out an inch or two. Put a pencil mark on the dowel right against the muzzle where the sight is (the twelve o'clock position).

Slowly withdraw the dowel, letting it rotate with the rifling, until the mark has rotated 1/4 turn (either the 3 o'clock or 9 o'clock position, depending on left or right twist in the barrel). Stop after 1/4 turn and put another mark on the dowel right against the muzzle.

Measure the distance between the two marks, and multiply by four.

You can use a ramrod to do this, but I find a clean wooden dowel easier.