I will assume you don't have Cigraph's ArchiStair which will put a wall under the stair.
The easiest way I have found is to use a roof at the slope of the under side of the stair. Position it in a section with the bottom of the roof plane where you would like the cut angle to be. Then use SEO to subtract the wall above your SEO roof plane.
Gerald
"The simplification of anything is always sensational" GKC
AC 25-4013 USA, CadImage Tools
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