When there is a lot of tightness in the claf while doing that type of hamstring stretch, it can often be an indication of some binding down of the the sciatic nerve somewhere along it's path. Sometimes doing a sciatic nerve flossing technique can be helpful.
You can give this a try. Do your hamstring stretch first paying close attention to how you feel and your range of motion, then the flossing technique, then repeat the hamstring stretch to reassess. If it's going to help you should have more range of motion and less calf tightness when you try the stretch the second time. If your tightness or range of motion get worse, stop doing it.