[NTG-context] Use of em dash and en dash

Until now I always used a - (minus sign) to define a sub sentence or for an optional word, like: This is -now- not necessary, I understood that normally you use the em dash for this. But for only a word, this seems a little big. Could I then use an en dash or should I keep using a minus sign? In which other places should they be used? -- Cecil Westerhof