Carbonation Advice

Hi there,

have been lurking for a while, but need some specific advice...  I am on to my 6th brew, which is due for bottling.  I have previously used carbonation drops, but on my last brew I tried the bottling bucket approach where I dissolved 112gr of sugar in 1.5 cups of water, boiled for a bit, cooled, then added it to the bottling bucket followed by the beer (20L ).  I have just cracked a bottle (3 weeks later) and it is rather flat.  It was a coffee porter, and I was aiming for 2.3 carbonation level.

Currently I have 38L of pale ale ready to bottle and I don't want it to be flat (or conversely over carbonated)?  My last ratio accorded to beersmith and the John Palmer recommendation.  Any obvious errors in my previous bottling bucket approach?  Comments / thoughts?

Cheers and beers,

JumpR.