Take a look at this recent discussion:
My takeaway is that you should not take the intervals prescribed by XMB too rigidly or seriously. If you need to do longer intervals for you goals, do longer intervals, XMB will adjust. The upside is that XMB will calculate your XSS no matter how you do your intervals. The downside is that you need to reach outside of Xert to figure out what intervals you need to train, if you are not happy with the short microintervals, which Xert seems to prefer.