Hi Morris - We can talk points if you like, but for most BB trade types (and there are variations), the trader uses a fixed points target. BB current rules, v4, set entry at BB High + 2pts, target at BB High + 13pts, with stop at BB Low -11pts. The points gain or loss never changes from trade to trade. They are about 1:1 to each other so number of winners is an equally good way to assess the perfomance of the system as points.
The two-hour period was in the v1 BB rules for the FTSE, I started using 0800-1000hrs as the opening range. By experience doing that I have cut this to 1hr, 0800-0900hrs. I found that the hour from 9 to 10 did not make much difference in terms of allowing the market to set a meaningful range. On the other hand, the longer the period you allow the market to define its early range, the less time you have for price to hit your target before the close or you scratch the orders to avoid holding overnight.
That said, a lot of people using opening range breakout trading systems (BB is one of these) use less than an hour. I think it depends on the volatility of your selected market, the 'work' the market will have to do to hit your target, and your lifestyle - I can be available every day at 9 to set orders: I can't so easily do this at 10. You say optimum and I think that is the right word, it's always something of a compromise and there is no magic number.