It's less mathematical and more psychological. People who don't want to be seen as compromising about that piece of information will use subjunctive even with a 99.9% probability. Sometimes subjunctive is just "I keep the option of changing my opinion". Other people will use indicative when probability is 50% just because they perceive you may benefit -perhaps, feel better- from hearing that.
Thus, being so personal, every person develops his/her own style. I have no doubt yours will weight probabilities, and that's 100% OK