Its a penalty. Lets take the LRM for example. Minimum range of 6.
Start counting hexes between your mech and the target. The hex next to you is 6. Count down for each hex closer to the target, including the target hex. Whatever number you are at in the target hex is your minimum range penalty.
Example: you are in 1205 shooting a Dervish with an LRM15. The Dervish is in hex 1209. Thats 4 hexes away.
Using the above method: 1206 is “6”. 1207 is “5”. 1208 is “4”. And 1209 is “3”.
This means the penalty is 3.
Another way: take the minimum range, add 1. Subtract the distance in hexes from it. Whats left is the penalty.
6+1 is 7. A unit in 1207 is only 2 hexes away. The minimum range penalty is 7-2=5.