How to obtain HU value

I detected a set of data and reconstructed the CT image through FDK, but the current grayscale value is in the [-3000,8000] range. I want to obtain the corresponding HU value to verify whether the grayscale value is correct, but I don’t know how to convert it to HU value because I don’t know the grayscale value of water and the linear conversion parameters of CT value? May I ask if there is a method provided in this ggems to help me convert to HU values,

At the same time, I discovered another issue that I don’t know why the reconstructed results show that the grayscale values of the skin are very low here. Is this related to my reconstruction algorithm? I tried FDK and the iteration results were the same. I hope someone can help answer this question