Measure the bore with calipers, and then mesure the ROT in the barrel using a couple of oiled cleaning patchs and a jag, that makes them a tight fit. Or, better, in my experience, use a bore brush, with a patch over it, that is oils. The bristles will punch through the cleaning patch, holding it to the brush, and the ends of the wires will dig into the corners of the grooves. Push the brush all the way down the barrel. Mark the rod( wrap it with tape to write on) so that your mark is in line with the front sight. Carefully turn and pull the brush out until the brush has turned 90 degrees. You can mark the muzzle with an 'X" before putting the brush down the bore. Now, measure the length of the ramrod from where it began with the brush all the way down, and where it is now stopped at the muzzle. Multiply that number by 4( since you only made a quarter turn) and that will give you the ROT for the rifle. You will usually have to do this a few times to stop muscling it, and get a feel for it. Otherwise, you can push the bristles over the lands, and get a way too short ROT. The typical .58 caliber barrel will be about a 1:66" ROT. I have no idea what your barrel is.
