I don't know how to work it out mathematically or scientifically but i use about 20g aroma hops plus steep hops. The only thing i have found with lots of aroma hops is that you really have to leave your beer to condition.
The amount of aroma hops also depends on the hops to a certain extent. I'm fairly sure you can't 'over do' aroma hops. Not without being silly. I use 50g Nelson Sauvin last 15 and 50g same for steep in my NZPA and it's just about right for me.
Start off with an amount you think should be ok and if it needs more add more next brew.
The amount of aroma hops also depends on the hops to a certain extent. I'm fairly sure you can't 'over do' aroma hops. Not without being silly. I use 50g Nelson Sauvin last 15 and 50g same for steep in my NZPA and it's just about right for me.
Start off with an amount you think should be ok and if it needs more add more next brew.