Just collect everything on an excel spreadsheet or get an app. Nothing will ever be 100% accurate but some can come pretty close and you can choose the exchange you'd like to use for your data.
Please read the opening post. It's obvious that an estimated value will never be 100% accurate. I'm only asking if anyone has any ideas on how to make the figure more accurate.
Is one exchange better than the next?
Is there a website that averages values from many exchanges?
Just any ideas that might help to gain some degree of accuracy.