How to get the value for line 13 on form 8860

[dmertz]

I'm guessing that you mean line 13 of Form 8960, modified AGI.

TurboTax calculates this automatically by adding to your AGI any excluded foreign-earned income, deductions allocable to your foreign earned income, and adjustments for certain CFCs and certain PFICs.  If you have none of these additions, which is the case for most people, the MAGI for line 13 is just your AGI.

For more information see Line 13 in the Instructions for Form 8960:

https://www.irs.gov/pub/irs-pdf/i8960.pdf