The standard expects the CBR for a soil to peak before reaching 5mm/0.2in. If it doesn't, re-check your work. If it really did hold up that long then use 5mm/0.2in.
The non-cohesive sand will interfere with the cohesive fines. The significance of the interference will of course vary between soils.
The plasticity only comes from the cohesive particles but total behavior includes retarding forces as well as driving forces.
