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One guideline is provided by the IRS -- the allowance for employees on business travel. Essentiallly, it is the cost of lodging and food away from home. Booklet 1542. One engineering friend a few years ago told his clients that his fee per hour began when he boarded a plane to leave and ended when he got off the plane when he returned home. After all, he was away from his business a significant amount of time.
Here's the intro to the IRS booklet.
This publication is for employers who pay a per diem
allowance to employees for business travel away from
home within the continental United States (CONUS) (the
48 contiguous states), on or after October 1, 2010, and
before January 1, 2011. It gives the maximum per diem
rate you can use without treating part of the per diem
allowance as wages for tax purposes. For a detailed discussion
on the tax treatment of a per diem allowance, see
chapter 11 of Publication 535, Business Expenses, or
Revenue Procedure 2011-47, 2011-42 I.R.B. 520, which
can be found on the Internet at www.irs.gov/irb/