To learn how far are the targets, look at the SAM launchers around the radar. Estimated range may be obtained by lookking at the map and see
where may they be in the direction of RWR signal (cities, airfields, known lines of air defence such as the channel...)
Btw, did you notice that when using ECM being not tracked they find you?
If what you mean is to engage, not to avoid SAM, use ECM, set them on some away from the area for 10-15 seconds. The noise will call their attention, as they track you (signal in the second circle of RWR), deactivate them and let them illuminate you for a shot (when the signal is in the inner circle of RWR) loose the ARM and break diving into the SAM site. The signal of a radar illuminating a target is the best for ARMs to find and destroy