According to RFC 6928, the SRTT is only calculated with the formula
SRTT <- (1 - alpha) * SRTT + alpha * R'
after two RTT measurements have been made. After the first measurement, SRTT is initilized to the value of this first measurement, ie.
SRTT <- R'.
You were close, according to the RFC alpha
SHOULD be 1/8 (= 0.125).
Note, in RFC's,
SHOULD means "there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course." (RFC 2119)