I think the sma length to use should be the same as the settings of the BB.
If the BB is set to 18 then set middle sma to 18, but standard default settings are 20 so if that is what you use then make sma 20.
If the BB is set to 18 then set middle sma to 18, but standard default settings are 20 so if that is what you use then make sma 20.