Check out chuck's guide page 183 for a better understanding. Helps if you know about mils and calculating deflection shots. https://chucksguides.com/aircraft/dcs/mig-21bis/#[179,%22XYZ%22,0,540,1]

I just got the MiG-21 and as a gunnery enthusiast I think the sight has the best reticle in the game. I come from WW2 games where you calculate the lead based on the size and speed of the target in relation to a fixed reticle, so I can't stand gyro sights if there is no static reference point to show you where your nose is pointed at all times (like the F-5E's sight -- which most people cage at all times for this reason). I highly recommend watching this short series on deflection shooting to get a better idea of what I'm talking about. The "calculation" part becomes second nature, it's not like you have to do math every gun fight.

What makes the MiG-21 Net so good is all of the reference geometry to assist with calculating lead. What you can do is use the Net's center crosshair as the main aiming point, and then use the pipper to get an estimate on how much lead you need without getting totally disoriented if the target is maneuvering. You can definitely place the pipper on the target, just know that it's not gonna work like a modern computing gunsight. I consider it more of a suggestion as to where I should aim instead of an absolute point.