HARM range in the sim is about 20-30 nm at low and medium altitude
(5-20,000 ft) and maybe 40 nm at HIGH altitude (30-40,000 ft). I usually
use 30 nm as a rough IN RANGE guide.
There is no way to know when the HARM is in range from only the HARM page
unless you are in PB mode, which assumes that there is a GPS coordinate
programmed in for a particular target. This is done during construction of
the mission and cannot be done by the Player during the briefing process.
To know if the HARM is in range, using TOO mode or SP mode, you can use some guidelines:
If the target is lobbing SAMS at you, you are probably in range for a HARM
shot unless the SAM in question is an SA-10 which has a pretty long range.
If you know where your target is (via WP or Target info), usually the highest concentration of SAM/AAA is in that vicinity, so you can use that knowledge to guesstimate the HARM range.
If you can use the 'click through the HUD' method of designation and get a designation close to the HARM box, you can then set your HUD to TGT mode and obtain a distance.
For ship targets, you can simply lock them up with SEA radar. When the HARM box is superimposed on the Targetting diamond, you not only know the range of the ship from you, but you know which ship is emitting. Of course, multiple emitters may be present very close to each other, but this will help you also in differentiating warships from non-emitting cargo and other (usually) neutral ships.