If construction work is still (!!) ongoing at Wolferton Pumping Station, the diverted stage 1 distance is 16.90 miles (otherwise 16.32 miles). You can easily amend this on the downloaded calculator

You can download this race time calculator from here. This will download as an excel spreadsheet which you will have to extract from your downloads folder. The spreadsheet contains instructions on how to use it together with a demonstration sheet which has been completed