Not sure about what you want to do but I guess you simply need to use "Ans".
As soon you press enter, the result is stored in "Ans".
You can use it like this :
Press "2nd" then "(-)" to get the "Ans" token. You can use it like a normal var (like "A", "B" etc...).
So you can reuse it and as you don't want to use a special var (even if there's a lot of vars "A" to "Z" !) you're ok with that.
But "Ans" contains the right value only in sequential opérations, so do not try to use a numeric input which contains "Ans" later or you will need to use history to redo the previous operation (the one to have the right value in "Ans").
Does it answer your question?