.I'm considering giving breech seating a try in my 40-65 and I have a couple of questions . Firstly I will only be using BP,  my Starline case holds approximately 67 gr of the 2f powder I use when filled to the top, this is more than I want to use, if I choose to use this much I will need a wad in the mouth of the case to prevent powder spilling into the action.
60 grains is probably doable if I carefully insert the case at an upward angle to prevent spilling. I figured that in either case I could use a wad of floral foam to keep the powder in place.
My question is this, using 60 gr and a thin wad in the case mouth would the air space left in the case cause any problems at all.
Mike.

I have a CPA rifle with both a .40-65 WCF and a .40-60 Maynard barrel.

My Weber breech seater is adjustable (switch case for the other caliber) to ensure I do not have an air space.
You can also use a plugged case to push in the bullet so the bullet base is even with the case mouth.

To use less powder just fill the unfilled case with wads to eliminate the air space.
A .060 wad will replace the void from the reduction of about 2.14 grains of powder.
So three .060 wads and 60.3 grains of powder fills the case with no air space - with the bullet seated with the bullet base against case mouth.

Swiss powder which does well without compression works well for breech seating black powder rifles.

I use a .060 vegetable wad to push down (even with case mouth) on a filled case of powder.

Best to have a tapered bullet to slide in easily with a breech seater.

Leaving too much of an air gap will result in ringing of the barrel.
How much of an air gap? Will depend on position rifle is shot (upwards) and powder load. 
So recommendation is to proceed with NO AIR GAP to ensure you do not destroy your barrel.
Rifles do not shoot accurately with a ring in the barrel - although they can be shot.

The .40-60 Maynard breech seats more accurately due to it's straighter wall.