You are asking about a tax write-off, I understand, and not an actual reimbursement from they company, yes?
To do this, first you need to start considering yourself a sole proprietor company: "Jane Smith, Stage Manager". You don't need to acutally incorporate or get an EIN or anything fancy. But you do need to start completing additional tax paperwork: the 1040 Schedule C (
http://www.irs.gov/pub/irs-pdf/f1040sc.pdf). Lines 9 and 44 deal with mileage, and you basically enter your totals and then multiply by $0.485 for your total reimbursement (see 1040-Sch. C instructions:
http://www.irs.gov/pub/irs-pdf/i1040sc.pdf). You need to keep your mileage record notebook in case the auditors come knocking one day. When you complete the Schedule C, you also need to account for all your income, and not just expenses you hope to write off, which means that the government is now reaching in your pocket to collect their taxes on your independent contractor gigs. But of course, you were paying on that already, weren't you, even if you didn't fill out the separate Schedule C??
While you're itemizing deductions on the Schedule C, you should also start tracking - in writing:
- supplies bought;
- postage for resumes, etc. mailed out;
- accountant fees for services related to the completion of this form; and
- insurance and healthcare costs (if you're 100% freelance with no other jobs that provide medical or insurance coverage).
The $0.14 "non-profit mileage" rate has to do with volunteer or charity work, not strictly miles driven in association with work for a non-profit business/theatre. You should check with your accountant, but I have always claimed the $0.485 for business mileage, whether my theatrical employer was a 501(c)(3) or not, and my accountants and laywers have agreed that this was correct.