Hi I would start in either Pobena or Castro Urdiales, they are both after Bilbao in the 650-670 km distance from Santiago. I think the total length is over 830km. Starting from these points would give you the chance to take it easy until you find out at what distance you are comfortable with, and if you get into santiago with a few days to spare then you can always head off to the coast to either Finistera or Muxia
There is so much variation in the distances given from the start in Irun to Santiago, some of this down to the usage of Arzua as the end point of the norte which does not take into account of the 40km from there to Santiago on the Frances route, also there is many variants on each days walking and some websites use different ones from each other, one example is mundicamino website goes via Oviedo which is 25 km longer than going via Gijon which Eroski website uses, the Norte is full of little variants which over its entire length can make up quite a total difference. There isnt a really good guide in English which will help you with this, you just have to go with how you feel on the day and which set of arrows appeal to you most.
You can use the little guides you get from the tourist offices to give you the outline of the main routes, and also when I passed through Pobena recently the hospitalero had the guide which covers all off the route to Santiago and also the Primitivo, it is a little booklet with very useful distance maps for each stage, its normally free but he was asking for a donation of 3 euros. There is a link knocking about on the Norte threads for it so if you find it you may be able to download it.
Good Luck
Mike