I was told by a cue maker that a cue's tip should come to the bottom of your lip when it is placed with the butt on the floor.
That is a popular opinion but in this case I need 173 sm (68" cue)

I'm the same height as my favorite basketball player mr. Air and I was able to play with a 60" cue for all my years in the game except the last two years when I started to learn the new for me process and craft and built a 62" cue equal split.
My 31" ferrule less shaft stays perfectly straight and it is stiffer than most maple shafts I've tried. Only two shafts I've tested felt stiffer. They were Revo 12,9 and Cynergy from Cuetec but I've tried them standard length on my 31" butt. Revo 12,9 on my butt felt extremely stiff...I've never tried something even close to this combination plus I know for sure the butt does make a DIFFERENCE too.
Just two days ago I've tried BK Rush of one of the kids from our school who is very lucky to have a great Mom and such Christmas present. I've made just two breaks 10-ball. One with original BK Rush and one with its shaft on my butt. The difference was huge. To tell the truth the break with the original BK Rush felt like no more power than if I break with this combo: my butt + the hard mapple shaft with fairly stiiff wooden insert at the joint. This shaft I've built for my student's break/jump cue being not fatter than 13mm at the tip weighs 5 oz and it performs much better on my playing cue butt.
But the combination of Revo BK Rush shaft on my butt really impressed me after the first shot.
Sorry for going in the other direction.
Here I can show two examples of me testing the Cynergy shaft so that combo my 31" butt plus 29" Cuetec's shaft makes a 60" cue
https://m.youtube.com/watch?v=WUEdkzI8QSE&t=206s
and here I'm playing with my regular 31" maple shaft on my butt = 62" cue
https://m.youtube.com/watch?v=PYKKq_Fk4Fo&t=5s
Anything shorter than 62" feels short now in my hands and even with 62" cue my grip is most of the time at the very end but I feel ok with it because the butt is wrapless. I might think on building an extension for some shots to reach but usually I just switch hands in those cases).