Can I calculate someone elses AGI from there w2?

The best place to get this number is from his originally-filed 2017 federal tax return. You can’t get it from a W-2, 1099, or other income statement. If you don't have a copy of your 2017 tax return, he can request a copy of his 2017 transcript from the IRS website which is available the same day. The IRS transcript will list the adjusted gross income or AGI.

• Rounded your AGI to the nearest dollar—be sure to round up when it’s 50 cents or more and down when it’s 49 cents or less.

For more info, please see this TurboTax FAQ, Can I get a copy of my tax return from the IRS? https://ttlc.intuit.com/replies/3301999

Also, please see  this TurboTax FAQ: How do I find last year's AGI? https://ttlc.intuit.com/replies/3301618