Manual XSS for MTB race

I don’t have a power meter on MTB, and have completed a 90min full out effort XC Race. I note from the manual calcs it states that ~100 XSS per hour at threshold, but given tiredness still from two days prior - should I factor more XSS for MTB ?

XSS doesn’t account for tiredness at this time. Only power , MPA and your signature.

I will use tiredness slider in addition in this case.