BFV uses hipfire sensitivity to calculate all the other ADS sensitivities, so if you don't have a hipfire sensitivity, it won't have any reference for your ADS sensitivity. In case you're wondering, the calculator always uses the input sensitivity (Fortnite) for all the calculations, so you can set literally any hipfire sensitivity in BFV, but if you are converting to 1X00 sensitivity, it will always feel the same. E.g.: with 0.009 hipfire sensitivity (BFV), your 1X00 sensitivity is 0.334158, but if you set hipfire to 0.001, 1X00 is 0.779702 and both of them will feel the same.
One more thing, since we need a hipfire sensitivity for BFV, it makes sense to conver it from your Fortnite hipfire sensitivity so it feels pretty much the same.