Which line on 1040 form is the MAGI ?

There is no single definition of Modified Adjusted Gross Income (MAGI); instead, there are several different, specific formulas tailored to different tax benefits, generally calculated by taking your Adjusted Gross Income (AGI) and adding back specific deductions like student loan interest, foreign income, or IRA contributions.

Do you need the MAGI  for the new 6,000 Enhanced Senior Deduction?  See Schedule 1-A top lines 1-3 to figure the MAGI for Schedule 1-A including tips and overtime. 
https://www.irs.gov/pub/irs-pdf/f1040s1a.pdf

Your MAGI is not readily found on your tax return (form 1040).

It is your AGI on line 11a, modified by the addition or subtraction of certain tax items, depending on the specific tax benefit you are applying for.

Please read this IRS document on the calculation of MAGI for different tax benefits.