Calculate tessitura for given stave

It would be very useful to be able to have the tessitura calculated for any selected stave.

Working on a musical, I have to give the tessitura to prospective singers.

I often write parts that need a versatile singer with a very good range. If you follow the guides (which vary of course!) they are quite conservative in "given" ranges. So I'm having to go through manually checking highest and lowest notes. A simple click would be great - can't be much involved in finding highest and lowest in a stave - or can there?