Hi, all depends on what you need, so an example would be more practical.
You'd get a percentage this way :
= (count(if(machine = 'yourdefinition', 1, 0))/ count(test))*100
Beware that you need to define the last part well. You need to make sure you get the right figure. In order to do so, I typically create several text objects, in which I put the first and the last part separately and the formula. That way, checking is easier.