Business trips to US

I'm just doing my first return that needs to deal with a few business trips back to the US while working for a UK employer.

From what I have found, Form 2555 cannot be used to exclude the share of your salary attributable to days spent in the US. However, a foreign tax credit can be claimed. So you might use the FEIE for most of your income and then the FTC for the US workdays.

Where I get hazy is around whether the US workday income is considered general category or re-sourced for FTC purposes. Any thoughts? It doesn't affect the tax liability, but just want to get the reporting right.

Useful link from HMRC for these purposes: http://www.hmrc.gov.uk/manuals/dtmanual/DT19886.htm