Proper adjustment of the tension is important for any optic but it's not always simple to understand how to do it correctly until you've had some practice. As a general rule optics should be hand tightened only, they do not need any significant force to lock into place. Having to push unnecessarily hard will damage the clamping mechanism so less tension is better than more. Optics should be snug on the rail and not have any front to back play though with the SVD/Universal castle nut style that type of play is more common than on the NPZ AK Universal style.

Here are some examples of how to adjust the tension but remember to take it easy and get a feel for what is good before making things too tight. Remember that no optic is designed to slide on the front of the rail, they all mount from the rear and slide forward. For more details on the main types of clamping mechanisms and how they work, go here

Adjusting the AK/Universal type mounts:

RS Regulate has a great video that also applies to the universal style AK mount:

The POSP manual from Kalinka also has a detailed step by step for the NPZ style AK mount. 

For the castle nut style (SVD and Universal) look at section 4.2 of this POSP manual

And for the RS Regulate AK300 castle nut: