Don't lose heart hon.
Personally, I'd hold off on the formula unless she's clearly still hungry after taking whatever she can from the breast. We've had some issues with Hol's milk production seriously dropping off, but perseverance and patience have allowed it to return to normal.
This is the same advice we got from my mum (a MW, with 20 years maternity unit experience behind her). You're clearly concerned over the feeding issue, and the anxiety over that could well be impairing your production, and even if not, your LO will certainly be picking up on it. Also, a baby is able to get much more out of the breast than even the best pump, so even if you can only express 1oz, that doesn't mean that she's not getting enough, because she's almost certainly getting more.
Ultimately, she's gaining weight based solely on what she's getting from you, either direct from the breast, or expressed. It may be slower than your MW/HV may like, but as long as her weight is going up, she's ok.