Do as Roger and Jerry suggested . That "Extra" S-Hook is going to bite you in the ass after a few cycles of the deck on and off . The exact spring , with the exact locating point is what takes care of the belt "back-lash" when turning on and off the pto . It wont take long before it throws the belt off and chews it up . It will happen most when turning off the deck at max engine speed , but will be subject to do so , at any time .
To correctly install the belt and spring :
Put the spring in first (no S-hook) - with one pulley not being covered with the belt , then take a lever or suitable tool to relax the belt tensioner as it stretches the spring . All the while - rolling the belt over that last end pulley . Be sure that the spring hole in the deck is not too worn to keep the correct tension .Side note : Your added S-hook would be the equivalent of the spring hole at deck being worn 2-3 inches .
I also think the belt spring tension mentioned in the above specs , is probably (to mean) 24 Lb.Ft/in. for initial belt deflection , pulling at a specified point along the belt travel . Then after belt runs in , it drops down to an acceptable 15.280 LB./Ft at 1 inch belt deflection . I can guaranty : It would take a heck of a lot more than 15 or 24 pounds to stretch that spring one whole inch ! There is a special spring device that tells the deflection of the belt , directly related (by proxy) to said spring tension , that reads LB.Ft @ (insert > belt deflection travel < increment here) belt deflection rate . It is much like a simple fish scale can tell how much a fish weighs , only different .