Surely it's just a matter of setting it out logically

Not knowing the layout of the building, it's difficult to tell you exactly what to do.
Therefore, would the most logical way would be to set it out floor by floor? Eg. Ground floor = # windows, #sq ft walls (approx), skirting, carpet, etc. . .
Then repeat for the other floors, with an overall one for any shared (such as lifts & stairs).
As it's only the communal areas, I don't suppose your client would be expecting War & Peace

Then just divide the price you have quoted accordingly. Simple spreadsheet should do it.