supplementing/swapping to formula

My 10day old currently gets one ff in the night to give my nipples a rest (bad latch). But she takes only 1.5oz and then still needs boob to pacify. Is 1.5oz enough and how can I get her to settle without boob? Although we are getting her tongue tie fixed I need to move to ff for my emotional wellbeing as I find it agony & so stressful.