We had an issue, when we were measuring LOS, we had a ship directly along the line, and we were not sure, if that would leave the ship in or out of LOS. We had te same issue with range, if you are exactly on the line, are you in or out of the range?.

You have to be inside the Firing arc. If by some miracle the line, as best you can determine, only touches the edge of the base you are not in arc.

Same for Range, If the Range band divider only touches the edge of the base you are not in that Range band.

Any part of the base counts for arc and range.

If it's borderline where as either cam determine or agree it's best to roll for it

Try using a laser pen to check..If it is real close and you still cant decide just roll red dice Hits or blanks(including eye) to decide.