When I started flying the R22 we used to have dipsticks for it, they looked factory but maybe the mist of time are working against me on that but they were T shaped and you dropped them through the filler neck and they would rest on the top of the T, now you would pull them out and hey presto you knew how much was left. We had one for the big tank and one for the Aux tank. Maybe banjo24 can back me up?