Yes, basically a 640 on 2x looses half the verticle and half the horizonal resolution ... i.e. 75% of the pixels ...
So a 640 on 2x is a 320 etc.
Now the exactly magnifications and hence resolutions for the legacy Armasights are not necessarily what the product name says ... so the devil is in the detail if for some reason you want the EXACT resolutions, you will have to look at the manuals. Like my Zeus Pro 4x was really 3.7x on NTSC and 4.2x on PAL IIRC.
3x at night, for whatever reason, seems like a lot more magnification than it does in the day and, for me 2x to 3x is still an idea magnification for a 640 device, then I can double the magnification and still be plenty useable as I am like a 320 device. I can even go to 4x and be useable as I am like a 320 device that went to 2x. But that is about the limit for me.