How do I determine the amount of my cell phone bill or internet bill that is business related?

There is not definitive way to do this that is described in the IRS tax code, but the IRS does give you the following generic guidelines for many expenses:

• It must use reasonable business practice
• It must be consistent

One reasonable way to do write off cell phone expenses is to determine what a basic plan might cost that would be sufficient for work purposes, and to assume the amount of your phone bill that exceeds this amount as personal.  For example, your bill might be $110 per month, but you determine that a plan suitable for work could be attained for $60 per month, then $60 per month is business and $50 is personal.  The same idea applies to your internet bill.

Other flight crewmembers have assumed a percentage that is not personal.  For example, they might consider their phone is 80% business and 20% personal.

There are other ways to do it, and it is up to you as a crew member to determine what reasonable is.