I found the width that I was extruding at by playing with speeds using a simple g code programme until I found that 150 mm/min onto gaffer tape was the best.(MDF and perspex did not stick and I was not using a raft)
I then extuded several lengths and measured the diameter with a vernier and found that it was 1 mm (the same size as the hole on my nozzle). I put these into skeinforge and then made the extrusion height over diameter 0.9 and these hearts came out.
As they are only 1 layer thick at the bottom there are small holes. To improve this I will make the walls thicker which will force Skeinforge to put 2 layers on.