Spine/Stu miller calculator question

[jerp]

I've got a new bow on order so I've been messing around with Stu's calculator to get me in the ballpark for what arrows might work best. Here's the question: the calculator shows a considerable difference - spine-wise - between using a 50gr insert and a 150gr head, vs. an aluminum insert and a 200gr head. Can just the extra 15 grains from the aluminum insert make that difference, or is it something about having 100 grains right at the end of the shaft with the heavier insert?
(yes, I'm over-analyzing, but that's the way I roll)

[Mike Javi Cooper]

His calculator uses the length of the insert beyond the standard length as well as the weight. So the longer insert will stiffen the spine where the short standard insert won't. similar to a footing over the shaft.

[chackworth3]

You have to insert the length and weight of the insert under footing and put yes for footing and zero for insert weight.