I couldn't tell you how the settings in the app might relate, you at least do not mention the sub app beginning level before changing it to -15? What's the scale?
I generally do not make adjustments to the sub's amp gain level once I set it in the avr, I make any increases/decreases in the avr. I generally add 2-3 dB to sub level trim in the avr and leave it there...but sometimes, altho rarely, revert to the base setting, depends on content..... altho in the end if what you've got set is pleasing, go for it.