Calculating area from a picture taken from a phone

Question

So basically we are working on leather image processing and we need to find area of an object using a picture in python. We have achieved an accuracy of up to 93% but we are willing to make it more accurate. I would appreciate if the stack overflow community could help me out in a better approach to find the area of the object detected from a phone picture and it's conversion into millimeter. We have already tried the reference object method but it's giving accuracy till 93%. Attached are sample images; enter image description here