You need to lock the inference. Which means you want to keep the octagon orientated to the blue axis. To do that, position the tool so that it inferences to the edge and is blue, then hold shift and move the tool up to the position you want it and click to start the shape and release the shift key and drag the octagon to size.
Quicker to make a vid than to try to explain more.
In the vid I move around to show the different inference colours, then when down the bottom and blue I hit shift to lock the inference.
[screenr:2a7024dw]RGHN[/screenr:2a7024dw]