Network Congestion and available bandwidth estimation?

Could anybody please give me some idea about this please.

If we take maximum bandwidth of a link is X

And the current bandwidth is Y

Then the available bandwidth will be Z = X-Y

Is the available bandwidth estimation correct?

2.

We know,

If the network is congested then the bandwidth decreases significantly with the increase of RTT (Round Trip Time).

So, when the network will be congested that time available bandwidth will increase. Is this correct assumption?

Thank you in advance.