Gauge and the garter stitch

I can’t seem to get the gauge right. Pattern calls for 18 sts 40 rows per 4 inches - Garter Stich. When I get the 18 sts, I’m short on rows. Because the pattern has mitered corners, I have to have BOTH sts and rows match the gauge. How do I do that? Thanks. Jen

PS the pattern is for the “Mitered Pearls Jacket” by Prism

Usually garter st rows are double the st count, that’s odd that you would need 2 more rows. Try going down a needle size, that usually affects rows more than sts. I think you could also try it as is and it shouldn’t be off enough to impact the corners.