A little more detail on the building methinks!
rather depends on what you consider to be a big commercial job.
I would personally consider a big commercial job to be one that would take a full day or more to clean.
A medium sized commercial would be one that might take up to 5 hours to clean (WFP, not Trad, if trad then I would consider a medium size commercial to be about a days work)
Big commercials tend to be much tighter on price than medium sized ones, those in charge of the money will think nothing of paying out £100-150, but will think very carefully about paying out several hundred a month.
For the big stuff, break the work down into units that you can charge...say...75p per unit [window]
Whatever system you use that will cover you, and should also be competitive.
Increase that if you are going up 3 floors though [for the 3rd floor of course]
On a medium size commercial you can crank the price up.
For instance I picked one up yesterday that took me 3 hours to clean, I charged them £110.
Hard work this time as it hadn't been done in 2 years. But the next clean will be done in about 2 & a 1/4 hours.
I couldn't hope to match that earning potential on a big commercial job.
On a big commercial I would have to charge a day rate, and providing I was guaranteed payment on completion that would be a minimum of £150. [per person too]
if it is a case of billing and waiting anything up to 3 months to get paid (trust me, it happens) then I would go in with a day rate £50 higher.
Ian