Your change in temperature has a very huge range, lots of error.
As to if you are correct in your calculations. I dunno.
"about 0.1g Mg" is very vague. But the hardest part is "about 50 mL of HCL". First, it is HCl, second about is vague, and third unless we know the concentration of the HCl we have no clue.
Also, show your calculations.
And, you are writing a balanced equation and accounting for the stoichiometry of the reaction, right?