I've posted this in the ideas section, but thought I would get the opinion of people on here - I can't seem to get it to constrain to a direction easily, and it always seems to prefer horizontal (depending on object space direction). It's so useful, but I wish Autodesk developers put some time into getting it working like you would expect it to:
https://forums.autodesk.com/t5/3ds-max-ideas/quickslice-drastically-needs-improving/idi-p/12703114
Snapping to hor/vert direction with modifier key would be nice. Meanwhile you can try to snap to grid for straight cuts.
I actually just create a plane, snap that to the vertex I want to start with, and use it as a virtual ruler (snapping the quickslice to the bottom vertex). It's not ideal but the only way I can get a straight vertical cut approximately from a vertex.
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