Question

how the value of the first I.E of Mg would compare with the corresponding value for Ca?

Answer 1

do you know how ionization varies along the group from top to bottom

Answer 2

It would be slightly larger for Mg (they're both in group 2) because Ca has a larger radius and so it's valence electrons are held a bit further out (less effective nuclear charge experienced) and so "easier" it is to ionize.