Yes, this is exactly how it works. It depends on how many nights you're likely going to stay next year.
By the way, in the UK you can get the Marriott Rewards Credit Card which will give you 10 bonus elite nights every year (i.e. you'll only need 40 actual nights for Gold since you'll get 10 bonus nights for having the credit card).
If you think you'll stay 40 nights next year, I'd apply for the credit card and stay 75 nights. Otherwise go for 74 nights and apply for the credit card in January.
One more caveat I see: I've read that Marriott has given Platinum status to people who haven't reached 75 nights but got very close to it. If this would happen, it would obviously defeat your plan. I'm not sure how likely this is though... maybe others can comment.