Bring back the Dalton computer & chinograph. I've no idea how it did it, or what the formula was, sorry, but it worked. Have any 4 from 6 and calculate the other 2: long before pilots became dependant on 'japanese brains'. Indeed I've never seen formula; it was pictorial on graph paper with ruler & compasses on a desk, or the DC slide on your knee as you did your nav-ex.