Indexing numbers with decimals and no extra 0s

When you insert the index marker contents, enter the desired sort order in square brackets after the content, i.e. for 121.3 [121.003]. Then FM uses the entry in the square brackets for the sort, yet displays the other content.

re: What I want is that 121.3 is not 121.300 (which it would be as a number), but that it's actually a 3 and it would come way before a 113. Without adding unnecessary zeros (i.e. 121.003), is there any way I can get these in the order I need them?

Quick fix would be to use "121.003", and tag the "00" as Conditional Text (say, "HiddenLZ"). Generate the document with that condition enabled. Switch it off to render.

I very rarely do anything with indexing … but is the second part of your question related to Group Titles?

Have to say that if I was an innocent reader looking for 121.113 in an index, I'd expect to find it just where FM puts it :-} but I appreciate that regulatory requirements/references often seem to have been devised by a committee in a different galaxy :-}