So get up, stretch, go to the bathroom, but use your best judgement. FA's can't force you to stay seated as there are many people with medical issues that require them to be mobile after sitting for a long duration.

The airline has done their due diligence from a legal perspective by keeping the seat belt sign illuminated and since you have stated you have the ability to accurately predict turbulence, you should know when it's appropriate to move about the cabin.