I tried to create a batch file, using a resize operation on to an image that is 720x720 pixel. I need to use a factor of 75% for the width (works fine), but the width has to be rescaled with 75.56% (I do not know how many digits xnview supports, but in case -> two digits after the (.) would be enough). If applied the needed result has to be 540x544, but the after the batch, the final image is 540x540 (I uncheked the "keep ratio" dialog), so it seems the digits after the (.) are ignored.
I also tested it by opening an image and used "image -> resize" and switched to %. There you can see, that image does not change its size by chaning the digits behind the (.). The Image size does not differ, no matter if I take 80% or 80.7% for example. (I also unchecked the "keep ratio" dialog)
The only way it works, seems by adjusting the "print size" other thant "screen size". Here I got the possibility to use 3 digits behind the (.), but when using the batch option, resize only offers the stanard "screen resize"