What's the reason you need to calculate it specifically? If you need to know what time you'll arrive in Cambridge for onward meetings that's one thing. However as for picking a train at Kings Cross, you don't need to in advance. Just buy a standard off-peak return - the price is the same irrespective of when you book (£40.80) - and is valid on any train. There are no reservations. The fast trains leave every half an hour from Kings Cross.