I think the subaru spec is the rolling circumference needs to be within 1/4". Not sure how that works seeing that my car came with a donut that is about 1" shorter than stock.
Do yer math and see if you're good. I think it's about 1/8" of tread wear will keep you in spec, which is about 20% of the life of most 10/32 tread tires.
C=2*pi*R