Support Contingency on Release Capacity

I'd like to be able to specify 20% contingency for a release. Meaning that if I have 100 points capacity, 20 of those points should be reserved for contingency / iteration (e.g. if doing Agile / Scrum). I'd like to see the contingency visually on my capacity planning bars and how much contingency I'm creeping into. So I'd like to be able to fill the release with 96 points, but to see that I've already allocated out 16 of my contingency points, not leaving much room for iteration during the release.