how to calculate common area maintenance?

Answer 1

These are generally covered by the leases.

But, are you asking what is an equitable way of determining what it would be? Let's see.

If I were doing it, I'd use square footage of each unit. Let's say you have a strip mall with three tennants. Tennant 1's unit has 2,500 square feet. Tennant 2's unit has 1,500 square feet and 3's unit has 1,000 square feet. That's 5,000 square feet under lease.

1's assessment would be 2500/5000 or 50%.
2's assessment would be 1500/5000 or 30%.
3's assessment would be 1000/5000 or 20%.