that rule of thumb is just that

variations based on body type are OK , as well as what makes you shoot consistently,

when I started years ago in Service Rifle, I had a guy show me some pointers in shooting sitting, (back when you had to drop into the position, )

he showed me, then said, don't try to look just like me in position, but this is the basics,

he is 6" taller, and skinny as a rail, so his lanky position was slightly different, but the basics on how to get into position were the same