As simple as possible laid off, the Mathis equation goes as follow
Rate <= (MSS/RTT)*(1 / p)
This is the Maximum Segment Size, which is the MTU excluding the TCP/IP headers.
MSS = MTU - TCP/IP headers - for example 1460 with an MTU of 1500 (20b IP and 20b TCP headers)
RTT is the Round Trip Time as measured by TCP. The round trip is the time it would take a packet to travel from endpoint A to B and from endpoint B to A.