I assume you are using particle board treads for the center runner and false treadcaps and risers. Usually the particle board tread and the false treadcap nosing are 1 1/8" thick. What you have to do is build up under the false tread cap (Because the piece of flat stock that is attached to the treadcap nosing is only 7/16 of an inch thick) so it flushes out with the top of the particle board tread and the front of the nosing. The same goes for the carpeted riser and the false riser.
That means you have to end the carpeted tread and riser where the false treadcap and riser starts. If you don't have a double jack where you have to make the splice you will have to straddle 2x4's between the center jack and the outside jack.
Once that has been accomplished adhere your moldings under the treadcaps. Because the moldings that are supplied with the treadcaps are only so long, purchase some cheap hem/fir moldings that are close to the same shape and continue on with them under the particle board tread.
The reason is once you have carpeted the treads the lump under the carpet is consistent with the applied moldings under the treadcaps.
The secret is to have the
Carpet Layer nap the carpet over the tread cap by one inch so it gives the illusion that the tread is a solid piece of wood all the way across.
Hope this helps? You can find out more about me if you go to my profile page or google
thestairguy on the web. I've got more pictures of my work and some write ups. Good luck and if you have more questions email me. ROLe
The Stairguy.