The best way would be to substitute by weight: remove X grams of sugar, replace with X grams of syrup. However...
I think your real problem was with the brown sugar. Older brown sugar can develop lumps in it that don't cream properly, and stay as hard chunks in the finished cookie. I look out for these lumps while creaming the butter, and crush them with the back of a spoon to break them up. If your sugar has a lot of these lumps, you might consider either sifting it before use, or blitzing it in a food processor.