There is a simple technique for chopping to a line with a chisel. This is important when dovetailing but comes up in other situations as well. Look at the picture below. I left quite a distance between the chisel edge and the line I'm going for (I should have left a bit less, truth be told).
Now look at the same chisel in the same location, except now I've given it two sharp raps straight down (the chisel is vertical).
The chisel is now touching the line. The bevel of the chisel forced it 'off course' and compressed the wood against the face of the chisel. Pull the chisel out and chop at an angle to remove some material as shown below:
When this wedge of material has been removed you will now be able to set the chisel in the cut again, but by angling the chisel 5 or 10 degrees over the waste material you will be able to hit it hard with the mallet to give a slight under-cut, but not corrupt the edge you have already chopped in its correct location.
When you do the same thing from the other side you'll have severed the piece and have two true edges on it. Lovely.