I now just use an in-line style statement, as it's easy to add with sigil's find / replace engine ( which retains recent code)
so I find a <p which follows a </h2> and replace
e.g. find
h2>\s*<p class="calibre2"
replace all
</h2> <p style "text-indent:0;margin-bottom:0.3em " - ( my preferred line space needs to be in there also )
less neat - but quick to do. the number after calibre varies from book to book.