Gearbox mainshaft wear limit

Hello, does anyone know if there are any measurements for the wear limit of the gearbox mainshaft where the sleeve gear bushes run? I can feel a slight reduction in the shaft diameter. 
Also I've read in our technical section the recommendation to change the layshaft ball bearing to a roller bearing, with mention of the requirement to shim the shaft to roller inner race to achieve the correct end float, what should this be?