From: Utopia Planitia Fleet Yards
From the manual:
Map Effectiveness Recovery — Map supply levels increase the effectiveness recovery of a unit. Headquarters also increase the effectiveness recovery of a unit. The base recovery is 6%. Each map supply level adds 2% recovery. A unit within the range of a headquarters adds another 4% to its recovery. When recovering effectiveness from map supply it is subject to a modifier
Effectiveness Modifier — The total of supply is discounted by how high in effectiveness a unit is. The less effectiveness it has, the more it recovers as part of the full amount. This reflects normal wear and tear usage of supplies and parts, soldiers getting ill, errors in logistics. The amount recovered from map supply is calculated as such.
(1 – current effectiveness) X map supply recovery amount.
The US 6th Corps with 60% effectiveness in main supply map level 9 with an HQ in command range would get a base of 6% + map level of 18% + HQ of 4% = 28%. Applying the formula (1 – 0.60%) X 0.28 = 0.11 = +11% effectiveness recovered.
Using a supply truck, beachhead, or air transport supply option on a unit gains a base recovery of 6% and +1 unit supply to the raw effectiveness score. A maximum of 34% effectiveness may be recovered per turn. A unit can benefit from only one HQ per turn.
After calculating the US 6th Corps map supply it recovered 11% effectiveness for a new amount of 71% effectiveness. They Allied player decides to use a supply truck to further increase it’s effectiveness. Now the US 6th Corps has a 77% effectiveness as supply trucks, and naval supply, recovery are added directly to effectiveness.
Chancellor Gorkon to Captain James T. Kirk:
You don't trust me, do you? I don't blame you. If there is to be a brave new world, our generation is going to have the hardest time living in it.