I've seen a description of how to make one using a spanner I think, plus a known weight hanging off it (think it's in the Haynes manual perhaps). Alternatively, I think about 10mm deflection on the long run is a rule of thumb but not entirely sure. A general rule of thumb which I have seen and used on Aux belts is that you should be just able to twist it 90 Degrees on the longest run. Not totally sure if that also applies to timing belts though.
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