Here's a post from Phil Knight, who guided the BDB development:
If you are using the stock baskets, then 17g will be a little on the low side as you will bottom out the tamper against the tapering walls of the basket at that level. 18-19g is what I've tested as the "optimum" level with the stock baskets. [snip].... adjust the grind until you see espresso start to drop from the spouts at between 11 & 14secs. If this happens then I guarantee you will have a good shot that runs from 30-35secs inc pre-infusion delivering a 50-60ml double (providing your dose distribution was OK).
I haven't used the stock basket myself much (no particular reason), but it is well thought of by many BDB owners.