I see in the change log for 10.8 that you added dismount_height option to RideEntity and MountArmorStand.

where or how do I change the option?

What are you trying to do?

I like the change but I was hoping to increase the height by just a few more blocks, thought it was a changeable config setting.

Oh, I got you! Yes, it's the broom spell you want to edit, this command should do it:

/mconfig spell broom parameters.dismount_height 10

Or if you'd rather you can edit the whole spell with /meditor spell broom and look for dismount_height in the parameters.