Mrs P needs the laptop now so that's it for the day. I've done about 280 blocks linear (2200 blocks mined) and found 18 diamonds ! That's about 2.5 hours but I was already on L12 and didn't hit any caves, lava or water...very smooth in fact
I need to go about another 134 linear blocks South (1072 blocks to mine) to reach the same Z coord as the spawn. Then, providing I haven't screwed up my orientation, I need go about 280 linear blocks East (2240 mined) to get near the spawn..knowing my luck I'll end up in the middle of the desert!!!
There's about a 57 block vertical difference between the tunnel and spawn surface for you to play with. Have you thought about automatic lifts? Or possibly escalators if they do them? Or a scary 45 degree rail car ride!!
Let me know how and where you want it terminated, remembering the point I'm currently aiming at is in the middle of the spawn and you might eant it outside. Give me a coordinate if you can