On my 7.3.4.2, setting above or below spacing as %-units is misinterpreted and unit is understood as cm. Seemingly, %-units are accepted but when you reopen the style properties, you see the unit is cm. In addition, spacing is clipped at 99.99cm and the effect is limited to page height.
In addition, using %-unit is nonsense here.You’d like the spacing to be relative to font size. This is not the way %-units are computed. They are relative to the same parameter in the ancestor style. In this case, Heading 1 is a descendant of Heading which is itself a descendant of Default Paragraph Style. If %-units were allowed for spacing distances, you’d request a modification of the spacing in Heading.
Suggesting a new distance unit as font size may be a valuable feature request. Have your try at Bugzilla. Develop a convincing argumentation with a description of an example use case … and be patient.