Math problem, 1 out of 4 unit didn't get meter on time for electricty

[Mohit Madaan]

So, here is what happened. I have a 4 storey commercial office building in another country and 2 storeys out of 4 storeys were vacant.

We had 2 sub meters installed, whatever the electricity bill came from authority, we would look at the sub meter readings and divide accordingly. So say tenant A consumed 60%, he will end up paying 60% of common area as well.

Now, a new tenant came in on 1st of May but his meter wasn't installed until 11th May, we agreed that they will pay on pro rata basis.

I am stuck calculating the bills now, and will need some major math work.

Here is what i have

Total Bill Amount 75,256 (don't worry, its not USD)

Total units consumed 8752

Bill Date 29th April to 29th May

Vacant Unit in building - 106 Units consumed

Tenant A 1717 Units consumed

Tenant B 3541 Units consumed

Tenant C 2303 Units consumed for 19 days (11th May to 29th)

Total 7667 Units

Common area 1085

If i take 2303 Units and divide by 19 days, it gives me 121 Units a day and multiply that by 29, i will in total get more money from all tenants than combined bill, which is unfair.

Please suggest a best way to calculate.