Two taps: pick your wall width, then the style. You get how many battens, the exact gap, the cut length and how many boards to buy.
More options: exact size, board size, gap, top rail, cm
Measure the wall at the height of the panelling, corner to corner. The calculator puts a batten at each end and keeps your existing baseboard (about 5 1/2 in or 14 cm tall).