Re: Any specific setting recommendations for F15HP and Sierra Towers.
You could try the blue snowball mic with REW. Not sure how accurate/sensitive it is, but you have it and REW is free.
As to the cross-over, yes what sounds best and gives the best integration between mains and subs. If you can use your mic with REW, you can run a sweep and look how the speakers/subs interact in the crossover region. Smooth as possible. Again, I use 80 (and haven't tried 60) but it sounds good to me and "offloads" more of the bass to the subs than the mains. I'd not go higher than 80 (don't want to localize the sub) nor much below 60. You can use bass heavy music that you are familiar with to see which sounds better.
Your edit is correct. Play your cross-over test tone and adjust phase to maximize SPL.
* LG OLED65E6P, BenqHT2050A
* Anthem AVM90, Rogue ST100, VTV Pascal 7 ch
* Sierra Towers/Horizon (all ELX RAAL), S2EXv2 surrounds, HTM200SEx4 heights, Rythmik E15HPx2
* VPI Classic 1+ (VTA & Fatboy Gimbal), PE Eagle/RR, VAS NOVA, Soundsmith Paua, Manley Chinook, Bob's Devices SUT, SugarCube 1 mini
* Oppo 203 & 103D, EverSolo DMP-A6
* miniDSP Flex, Audiosensibility & Blue Jeans cables, Symposium & Isoacoustics, GIK
* For RDJ: Anthem MRX720, Sierra LX, Luna Duo v2 center, CXNv2, TBD