Yes, the correct formula for conversion efficiency uses water volume (actually water weight,) but you cannot use water volume and points per gallon to calculate conversion efficiency. If you want to use points, you must do it with wort volume, not water volume. The only way to determine wort volume at the end of mash is to go thru the correct formula that starts with grain weight, grain potential, strike water weight, and end of wort SG in °Plato. Most of the incorrect conversion efficiency calculators try to use strike water volume and points, which gives you a lower efficiency than actual. Ignoring the moisture content of grain makes your conversion efficiency look even worse.
Brew on