Thanks for the help BigAir - much easier when you know how!
mmm one other question!
An aircraft is flying at 1.3 Vs1g in order to provide adequate margin above the low speed buffet and transonic speeds. If the mass increased from 285,000kg to 320,000kg and 1.3Vs1g is 180kts CAS at 285,000kg what is the new 1.3 Vs1g?
Thanks again...