Turn the set gain to zero to start with and "set" to where you want it.
(this is a % of max possible boost your turbo will support)
Then adjust the gain(controls how quick or slow it comes on boost) untill you get it to hold the boost level to redline etc.
Once you have the boost running to what you want then adjust the 'set gain"(what pressure it will hold the wastegate shut till) up until you boost starts to spike and then just bump it down a bit.
But as warriorbrawl said if you setup wont allow it then no mater what you set the bc to do it will not hold boost.
Hope this helps