ok so if you used howitzers at least once you know how cancer it is to aim and hit accurately without spending a million years aim. So this is how I think range finder would work on ccnet.
Craft method: Spyglass in middle, a gps marker under and above the spyglass (enhanced crafting table) = x2 range finder
1st Method: when used to range finder on a howitzer it opens a map with a circle around marking it's attack range. Player can click on the map to select target area, the howitzer wont be able to hit 100% accurately however but will hit in a 5 block radius of that area. After the player select the target the map will close and the cannon will be auto aimed at that chosen area.
2nd Method: When aiming a howitzer and using the range finder, a beacon light will mark the area at the crosshair, light blue beacon is valid range and red beacon is invalid range. Click once to select area, click twice to confirm. After confirming the range finder will be used up (the beacon light will stay for 5s after using). And the howitzer will be aimed at the beacon light. Like method 1, it wont be 100% accurate.