Uncertainty Measurement

I am asked to indicate the uncertainty on the radius R of rotation of the plumb bob. Please see picture.
Basically, there is a vertical shaft, over which is attached a horizontal shaft and a plumb bob is hung over with a thread on the horizontal shaft. The bob is attached with a spring to the vertical shaft. In lab, we start rotating the vertical shaft until the spring is stretched and the bob’s end is just over a metallic indicator (a bar).
The diameter of the indicator was 0.3 cm.
We did this lab and my teacher corrected us. But he accepted different answers.
My friends just put a 0.1 cm uncertainty on the measurement on R. THAT’S IT.
What I did is that I estimated my pinpointing of the center of the vertical shaft by +/-0.1cm. Then, I added 0.3cm (diameter of indicator). TOTAL UNCERTAINTY : 0.4 cm

Which one is RIGHT?

Attached Images
File Type: png Screen Shot 2014-04-19 at 7.42.41 PM.png (24.5 KB)


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