I can't remember for sure but I think you had to do all or nothing if you were getting an incentive for all of the points. So if you're getting a 250 point incentive, you would have to do MB for all or none. If you were just purchasing 150 and 100 at the seperate incentive levels*, you could do MB however you wanted. Hopefully someone else's memory is better than mine!
* this might seem like an odd strategy but when they had the big VGF sale last year, for certain point levels it was a better deal to get two seperate incentives vs one on the bigger point total. It had to do with how they tiered the incentives. It was a flat dollar amount for multiple point levels instead of increasing with the number of points. There were sweet spots in the pricing that made it better to do seperate incentives (that was fun trying to get our guide to understand what we were looking to do to).