Didn't realise you were looking at the slack on your old belt rather than slack on the new belt you have just fitted...
There is a key way on the cam pulley, but the key way isn't a tight one if you know what I mean, it allows maybe 10 or 20 degrees of movement of the pulley to allow for tension to be evened out. As long as the camshaft & crankshaft are both held with the timing tools when you set the tension and then tighten the cam pulley back up, your timing should be correct. The cam sensor reads the back of the cam pulley wheel but it is only there to show the engine what phase it is on so its exact correlation to the crank pulley to the degree is not required. If it is outside of a certain maximum advanced/retarded position it will trigger an error but otherwise will show the engine which injector cylinder is on TDC at the start of the power stroke and which is on TDC at the start of the intake stroke.
I feed the belt onto the pulleys working around anti clockwise from the crank pulley, so over the idler first, then the cam pulley, at this point you can pull the belt tight which turns the pulley back (have the pulley turned forward first), feed it around the fuel pump pulley, then while you are holding that bit tight, slip the other end onto the tensioner and lastly over the waterpump which is the only un-toothed pulley. Tension, nip up the cam pulley, remove locks, turn it over a good load of times, refit the locks to double check nothing has moved, job done.
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