Hi,

over here 1.6 grain per mm barrel-diameter (Caliber) is regarded as a good point to start with on short guns ( pistols & revolvers). What means about 18 grain in your case. Personally I've found that those guidelines are a little bit to low but its better to start low and add, than to start to high.
Also there are some differences from label to label. I prefer Swiss BP for its fast, while others prefer slower BP for its softer whilst shooting.

Cheers Jochen