Crank Length 165mm or 170mm? mixed views I have been doing a lot of research into crank length, I have a 172.5mm crank which I find my knees come up too high, and sometimes I get a bit of back pain. I have a Scott Speedstar 2017.
I am 175cm tall, and an internal leg length of 78cm. I looked at the following video: 
                
             which was invaluable but, I find I am around 164mm for a crank when using the 9.5% of my height.
should I go down from 172.5 to 165? or just move down to 170mm? as was said at the end of the video, &quot;Modern bikes are designed around 170 to 175mm long cranks&quot;

should I goto 165mm or 170mm long cranks? I know I need a shorter crank, just how much shorter is the question :)

Thanks! :)
Percentile based formulas for crank length are trash. Femur length, health or special needs of all the joints in your leg, and the desired relationship between your saddle, knee, and pedal are all factors, as are the clearance needs of the bike, the type of riding being done, and Q. There is zero reason why a simple percent of your height would suffice.
Fitters who are serious about delivering a good result are mostly going to use some kind of system (or haphazard rule of thumb) to come up with an ideal saddle setback and knee to pedal spindle relationship, then choose a crank length based on that, then test it with either different cranks or a sizing crank.
As for your situation, if going to a fitter is off the table, you are going to be much better off making your own estimation of whether 2.5mm or 7.5mm is going to be the right amount of reduction. I would make sure you're feeling very confident about your saddle height being on point first, because that's a far more common source of issues with knees being put in an uncomfortable position.
Some shops do have sizing cranks and might be willing to set you up to spin on a trainer in a corner for a while if you're going to buy cranks from them afterwards.