Donated my car and can deduct the value of the 'arms length' sale price. What do I enter under "method used to determine value"?

Without a signed form 1098C, the maximum you can claim as a deduction is $500.

You can use the written statement if it contains all the same information as the 1098C.  Here is the form so you can check to make sure that you have all the necessary information.

https://www.irs.gov/forms-pubs/about-form-1098-c

If your written statement qualifies, you must enter the donation as a vehicle donation, say yes that you got a 1098C, and answer the required questions.  You must mail the original signed statement to the IRS after e-filing, using a form 8453 cover page that Turbotax should tell you to print out.

If you don't have the right information, you should still enter the donation as a vehicle, say you don't have a 1098C, and let the program assign a value of $500.  If you are still asked for a valuation method, say FMV or thrift store sales, it doesn't really matter since the $500 limit is in the law rather than based on the real value.