Long exposure... please feel free to critique!

OK so I managed to attach a filter and went off this morning to try to get some soft water. The slowest shutter speed that I achieved this morning was 1/10" and didn't look particularly impressive to me.
This afternoon I tried the darkest filter which I assume is ND16. This gave a shutter speed of 0.4" but was very underexposed. I increased the exposure by +3 in PS and this is the result. Should I have increased the exposure by +3 in the camera and would that have given a slower shutter speed and therefore softer waters edge? Or should I have used a totally different filter... any comments appreciated 'cause I haven't a clue what I'm doing... ;-))
Yes there is a speck of dust in the sky; I know I should have cleaned the filter!