I had been in the 'something else' (or manual) page and I had not considered the +/- options on the left side of the window. I'm a bit bit fearful of clicking anything in a partitioning program unless its clear what I'm doing. Since I had passed the caution stage and was willing to risk it I chose the - and then added clicked the + which brought up the box to instruct the installer what I wanted to do. Then as gohlip mentioned... Then select the old debian partition to install as root (/).
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