Essentially The Depth is from the manufacturer I have converted this to CM and also Litres the last column (column D) is what the watchman oil guard I have displays, it shows a 1 to 10 and I have worked out what they would be in the spreadsheet as well. I guess what I am looking for is a way to add the litres. Note I am not doing any calculation of the temp so I am aware this will always be slightly incorrect. I wish I could do it via a PHP script but like I said the dimensions of the tank made it way to tricky for me to figure that out.
The details I used to figure out the tank details are from these two websites:
https://www.fueltankshop.co.uk/files/pdf/pdf3558.pdf - Tank Dimensions
https://www.fueltankshop.co.uk/files/pdf/pdf3558_c.pdf - Fuel Calibration Chart with curve