I currently use 3.4g of Vectan AS behind a 158g bullet in a 357 case in my Marlin and this works great at least out to 50 yards. I've also looked at using CSB5 as suggested by mag44 as AS has become expensive. Being a careful Yorkshiremen I don't like to spend money on expensive powders when a cheaper shotgun powder will do the job. The last I heard AS was £60+ per kg and CSB5 less than £35 per kg (see mag44's posting). I dread to think how much N320 is these days. It is good stuff though.

For interest I've found the following:

3.4g AS gives 875fps

3.0g CSB5 748fps
3.2g 760fps
3.4g 838fps
3.6g 886fps

So 3.5/3.6g looks about right for me and confirms AS and CSB5 are not that different.
CSB5 is slightly denser than AS.
My powder measure is set to drop 3.4g of AS and without altering it it drops 4.2g CSB5. For comparison it drops 4.4g of Viht 320.
These loads worked OK in my Marlin but you'll need to check them out in your own gun to make sure they're OK for you if you go in this direction.