I'm not sure how many other mum's out there might cringe at this, but I do tend to give my bub a bottle of formula every now and then. Same as you I felt I didn't have enough supply after those hourly afternoon feeds, so started giving her the formula. I only do it every couple of days.
I spoke to a CHN when I was doing it, and she said it's ok to do for a top up, and as long as you don't start replacing breasfeeds with formula (as in during the day), you should have no problems.
Just a thought though, has your bub ever drunk from a bottle before? Expressed milk maybe? Not that this happened to me, but aparently some bub's don't take back to the breast well after formula.
If you want to do it, you can buy Karicare sachets (for about $1 each) that make up a bottle, rather than having to buy a whole tin. Also useful in case bub doesn't take to that brand of formula.
Best of luck,
Sam and Hayley and bub