Form 2555: Computation of income earned in US on business

Is there a generally accepted method for calculating earned income for "number of days on business" in US for form 2555?
For example, the scenario of a UK PAYE employee (fixed salary), spending a couple of days in the US on business, in conjunction with a few days holiday. 

Would the following be a fair example?: Divide annual salary by number of business days in the year to get salary per business day.
If so, should that be the number of Mondays-Fridays less UK bank holidays? Do the number of vacation days the employee has in that year get subtracted as well?

Yet another thing that isn't as simple as it seems at first..
 

In principle, you should be keeping track of your actual total workdays, so you know when you have taken a holiday on what was an otherwise normal workday or when you may have worked on a weekend, etc.

If you have not done this, then you need to make a best guess if you can't do anything else.  This really isn't ideal because, depending on your level of salary, a few days difference can significantly effect your tax liability.

In practice it usually makes little difference because for Federal purposes income attributable to US workdays is normally treaty resourced as foreign source income on the 1116 (not the 2555) so there is usually no US tax on the income after foreign tax credits.

There still can be State tax but this is creditable in the UK.

Thanks.

Now, to spell it out for accuracy's sake (because it wouldn't be the first time I've missed the point  ).. Total actual workdays are total days during which services are actually performed (and not holidays or annual leave days taken, and not even sick days if any were taken during the year)..   
The total salary divided by the total actual workdays would give a figure that could reasonably be used as the daily rate for form 2555, for the purposes of determining earnings during 'days on business' in the US.. correct?

Yep. 

Your foreign workday comp will be foreign workdays / total workdays x most compensation.  Certain comensation items do not follow this format (employer paid housing, hazard pay) but I won't go into detail and confuse you if you don't have any comp items beyond the basic salary, bonus, etc.

And as you said a workday is when you worked -- not sick, holiday or weekends.

Many thanks for the replies.

In my case, the small amount of time on business in the US ends up making no difference, because all income not covered by the FEIE still ends up being less than the standard deduction and exemption.  (Well, it does make a difference, but only due to making the forms a bit more difficult to fill out accurately.  )
Much as I hate how much the GBP has been devalued over recent months, at least this is one situation where it has a beneficial effect.