Best way to program

I have a large HWQS home I am programming (9 PNL-8, 40+ shades, 70 keypads)

The home has a exhaust fan recovery system. All bath exhaust fans are fed back to one location where a main exhaust fan puts the air back in circulation throughout the home. So, when turning on an exhaust fan in a bathroom I need to turn on the main exhaust fan. That's easy enough. When turning off a bath exhaust fan I must first check if any other exhaust fans are on and only turn the main exhaust fan off if no other exhaust fans are on.

I was hoping to share the logic via a scene/sequence but I don't think thats possible (or is it)?

Do I have to build a if then loop for every exhaust fan button?

You can program this one of two ways, both would have their own level of complexity where you can decide what would be easiest.

Method 1:
You can build 1 if statement and use "And" instead of "Then" to query for all fan loads.

If Fan load 1 is off And
Fan load 2 is off And
...
Fan load n is off Then
Main fan OFF

Method 2:
Create a keypad that will be link assigned but not activated/transferred in the project.
Add a single action scene with all the fans being off (the LED on this button will be on if the fans are off)
In the conditional, verify the LED of that keypad button

IF LED of virtual keypad 1 button 1 is ON
Turn main fan off