Originally Posted by http://www.hoi4wiki.com/Logistics
Incoming supply is equal to the bottleneck (lowest value) on the best path from the capital including both the end supply area and the capital. Supply areas are connected with supply values as follows:
By land: Supply areas with a land adjacency. The value is the sum of:
0.5 times the square of the average infrastructure in the sending supply area. Capitals do not receive this factor.
2 times the square of the average infrastructure in the receiving supply area. Note that capitals get this value as "local" supply.
By sea: Supply areas with a sea connection. The supply value is 3 times the total naval base levels in the sending and receiving supply areas, whichever is lower.