Home Office Deduction for a Shared Apartment

Our son is a freelance editor (1099 worker). He meets all the criteria for being able to claim the home office deduction.

He rents an apartment with 2 other guys. They each pay 1/3 of the total rent, utilities, etc.

If he uses the Actual Home method to calculate his Home Office deduction, is it ok to use 1/3  the square footage of the apartment when figuring % business use vs the sq footage of the entire apartment?

Just for explanation (not his real figures), if:

Total Rent = $3,000 p/mo

Apartment Total size= 1,500 sq/ft

Son’s Share of Rent = $1,000 p/mo

Son’s Home Office Work Space Size = 60 sq/ft

Can he claim:

60 sq ft Office Space/500 (Total Size of “1/3 of Apartment”) = 12%

so,

$1,000 X .12 = $120 (Son’s Home Office Deduction for rent)

It does not appear accurate to me if he calculates the deduction using just “his portion” (1/3) of rent, but then uses the “entire” square footage of the apartment.  ??  

i.e.

60 sq ft Office Space/1500 (Total Size of ENTIRE Apartment) = 4%

so

$1,000 X .04 = $40 (Son’s Home Office Deduction for rent).