Yeah those tangs can suck up your budget pretty quick, but totally worth it! :thumb:. If you just want three piles, I would say that 50kg will be more than enough, but 25 probably won't be enough(considering your tank is 70cm high.). Go with 50 if it fits in your budget, but 25 could be made to work if you needed it to! Something like 40 would probably be the perfect amount, I guess I don't know for sure though. Have fun decorating!
As an example, I have about 80 kg of limestone in my 75-gal tang set-up, but limestone is way denser than tufa, and the rock is stacked from end to end of the tank and from the bottom to about 2 inches from the top.