No pun in intended, but I'd say use your "head". An eight or nine foot ceiling needs no down rod. Nine feet? Nothing beyond six inches or a foot. On ten feet, I'd use only a foot and a half of down rod, On 12 1/2 I'd use two feet of rod. When you walk under a fan? You want it UP there, not whirring just above your head.: )

Just be sure you have a fan fixture box installed before you mount your fans. Sounds like with the ceilings heights you have in some areas the fan will be of substantial size. Check the fixture box rating, usually, if the maximum weight isn’t marked on the box, it means it can support a fixture up to 35LB and if the box marked with a weight limit, it can support a fan up to that weight, but it cannot exceed 70LB.

That being said, ceiling paddle fans that weight more than 70LB have to be supported independently of the outlet box.

Lucy, don't bogart Matt's post. Start your own.

Matt: Stand up; yawn and stretch your arms up; measure it. Take a sweater off over your head; measure it. Then add a few inches.

I'm 6 feet tall, and I am this close to being able to touch an 8-foot ceiling. No fan should be anywhere close to being in touching range.