[Fyron]

The threshhold is 250, not 150. Anything from 250 to 300 will drop to 249 after one turn.

You can only use three satellites if they are mining different resources (one of each type).

Multiple mining components on a ship/base do not increase the rate of value loss. It is 1% per turn if 1 mineral is mined or if 100000000 minerals (in a crazy mod) are mined. Each resource value can be decreased each turn if all resources are being mined. Technically, it is more efficient (for ships/bases) to only mine a single resource type at a time, as you will be able to mine the asteroid for 3 times as many turns (assuming each resource value starts equal).

If the satellite is in any sort of group, it is impossible to tell how many resources it is mining in any direct fashion.

Also, note that remote mining does not require the presence of a space port.