It took me awhile to work it out but I hope I have got it now! To work out the 70% hydration ratio for a sourdough loaf, it is necessary to calculate the amount of flour not the total amount of dough. Using Jacob's recipe of 500g flour and 500g of sourdough starter, the calculation is made on the weight of flour in the loaf, less the amount of water in the starter
1000g of dough = 750g flour
750g flour x 70% = 525g water
Less 250g of water already present in the starter
= 275g of water to be added
Salt 20g
However the amount of salt added 20g seems to be 2% of the total weight of dough!