Supplement subscription with paid points

When trying to process a recent image set I had about 420 images, putting me well over the image limit for my free account. I purchased a subscription, thinking the 400 image limit could be supplemented with a few paid points to get my 420 image map processed. Now when I prepare to upload the images MME is calculating points consumed at 750 after the size discount. I was under the impression that I could process 400 images with my subscription, and purchase points to augment the remaining 20 images (should burn about 320 points). Am I misunderstanding how this works? How does the augmentation of a subscription using paid points work? Also, how is the $.36/Gigapixel cost shown in the subscriptions specs calculated? If I use that number this map should cost me a couple of bucks, rather than the $15 I paid for 750 points. Please provide some clarity. 

Here's a screenshot of MME calcs:

Thanks, Adam