How do you scale your pep helmets in designer 3?

Greetings,
So recently I have been having lots of problems scaling different helmets in pep designer 3. It was a bit easier in designer 2 with the scale option in 2D menu that allowed you to scale the assembled height. But I dont see it in designer 3. So I have been using the measure from two points but its hard to get a really accurate height measurement.
So what are some of your methods of scaling helmets in designer 3?

The average height for helmet should be 275 -290mm anything else will be too smalk or too big.