After +6 it should be 64%, after +7 it should be 32% after +8 it should be 16% after +9 it should be 8% and so on. I would not like to see lots of +10 people running everywhere, You guys should make it difficult to obtain "Over-Powered-ness". If someone would like to get this "OP-ness" you should certainly create upgrade stones for different levels of tier upgrades.
Ex: After +6 you need a +7-+8 upgrade stone that adds 10% chance of success
This should be ideal.