There is very little difference between 100 and 125 heads dynamically. It might be an issue if you were weak on spine and switched to a 125 head. However if you were right on or a tad stiff I doubt it would effect anything. It would effect your FOC some, but you have plenty of that as well.

I do not believe your arrows would be "Way too stiff" with the 100 grain heads. If you have both I would try them both and see if it shoots better with one over the other.

Remember this is all estimation, even with a fancy program. Nothing is going to give you exactly what you need or garuantee that it will work. They will just get you close, and the programs are more accurate than the charts because they ask for more data. I had to turn my bow up 5 lbs to get the arrows I am using to bare shaft tune perfect. This is just a starting point, not a diffinitive answer. And carbon arrows use the same arrow to cover a larger span of uses, so you are going to end up with a stiff arrow depending on how your set up is, unless you alter arrow length or increase draw weight.

I need to look some stuff up and switch computers and I will give you some info on arrows and how spine works.

Paul