I'm sure clays is an Austalian powder made by ADI and called Clays on the US market, however its a fast powder in the same range as Bullseye and 231. You should be quite safe starting at around 2.5 grains with the 148HBWC seated about 1/32 out of the case. Crimp is everything to get the best accuracy. The only way to get the best grouping is to borrow a Ransom rest and then take all your loading gear to the range including a chrono. You can expect to spend at least 4-5 hours playing around till you achieve a good tight group. Remember the crimp plays a major part in accuracy, a quarter of a turn can make a huge reduction in the group.Also try a taper crimp Check the velocities with the chrono, if they are inconsistant then you will have vertical strings and a lousy group.
Be careful with HBWC pills with thin skirts as they may blow off if shot too fast giving yo two holes in the target or worse a skirt left in the barrel. Once you have get that tight group lock your dies and don't change them for another load, buy another seating die.