Instead of what RFC2581 describes, the initial congestion window is increased to 2-4 MSS:

min (4*MSS, max (2*MSS, 4380 bytes))

For the MTU of 1500 bytes, the MSS is 1460 bytes. Then the initial cwnd is 3 MSS. The initial cwnd setting is actually according to this table:

MSS max cwnd
MSS $\le$ 1095 bytes 4 MSS
1095 $<$ MSS $<$ 2190 bytes 4380 bytes
2190 bytes $\le$ MSS 2 MSS

Bibliographic data

@misc{
   title = "Increasing TCP's initial Window",
   author = "M. Allman and S. Floyd and C. Partridge",
   howpublished = "IETF RFC 3390",
   url = "http://tools.ietf.org/rfc/rfc3390.txt",
   month = "Oct",
   year = "2002",
}