I just measured the actual offset in inches vert. and horiz. and then did a true parallel zero, and then verified at distance. The primary was done around 40 yards and when I verified at distance it was dead on. Even if you get a co-witnessed (ie not parallel) zero at 1000, you won't have more than the absolute distance between the laser/sensor and scope center at the gun, at 2000 yards. The problem I had with using a far object to co-zero the LRF was that the visible laser on the Impact is pretty weak and seeing it at a distance was hard. You almost need a semi-reflective sheet to know when you're close, and then a very reflective small (e.g. 1") dot on that, at the exact linear up/down/left/right offset vs. the scope aiming point.
Using a red dot for aiming is a red herring and not really relevant to if the LRF is side or top mounted. For example, you can run a top-mounted LRF with a red dot at 9, 3, or 1:30 o'clock and the considerations for how you actually aim the LRF are the same. Or you can have a side-mounted LRF and not even have a red dot. They are separate topics. The biggest advantage of a side mouned LRF is the ability to read the screen without majorly disturbing the shooting positition.
Having a red dot on a gun set up with a "long range scope" is pretty useful, especially for gas guns (e.g. SPR/DMR) that might be employed faster or at a much shorter distance. Just from my experience with the setup, there are times when it was much easier to range using the red dot, and there were times when we had to aim with the scope.