A bent bar will make a nice curve..... just do it by your own judgement, hold the saw up level, and get the dogs to bite and turn the saw, as far as you can go with it without having the dogs let loose, then take the saw down to the end of your cut, when the slab drops hit the chain brake and pull it out or push the slab off.
If we have to leave a stump high for a customer we take the tree down then do it by free hand, if you dont get it right the first time just shave a little more off.
Ian
STIHL