What i experienced with the Mountaineer, is the bits of powder, and soot in the barrel, kinda act like a lubricant i.e. the bullet/sabot went down the barrel easier, after it was fouled. Never did i ever clean the barrel, nor the breech plug, whilst at the range. Can't for sure write how many shots i put down the fouled barrel during one trip out shooting, but seems to me it may be around 30 or 20. The breech plug with it's large volume flame channel, never required cleaning until i came home.
If it were me concerned about accuracy, i guess i would start with about 90g of powder, and keep adding until there was a noticeable difference in accuracy. To really 'know' the best load may require many many trips out shooting, and many bottles of powder, boxes of primers, boxes of bullets, and containers of sabots. All the rifles i take hunting anymore, are loaded with 110 to 115g of powder, because i hunt the great plains, and desire long range capability. For short range hunting, i would probably use 80 or 90 grains of powder.
The Mountaineer should be able to safely handle 140g of powder, and no cleaning of the barrel, or the breech plug, should be required between shots.