If you pull the belt up and eliminate the 1/2inch of slack on the longest side to stop the belt squealing under load, you risk pulling the water pump baring out, or the alternator. If you need to tension the belt tighter than the recommended 1/2 inch of free play, then the belts worn and most likely could be bottoming out. Time for a new belt and ensure its correctly aligned.
To add to that - use a decent quality belt - not the crap being sold by the various mini suppliers - buy a fenner or a powergrip or a gates, not a charley farley hoki koki belt.
We transmit huge amounts of power using V belts - we can shift 1 megawatt over a set of V belts with no trouble BUT, you can't do it with rubbish belts.
It's important to note that just because a belts fits over the pulleys, it doesn't mean that it's the right "section" belt for those pulleys - oh, and if you run the wrong belt, you can wear or worse, spread the pulley sides so the drive is compromised.
Check to see if the pulleys are all in good condition then check the alignment f all the sheaves for parallel and lateral alignment, then lastly fit a good quality correct section and length drive belt before tensioning correctly.
Ohhh and if you've run the belt slack for a while - bin it and fit a new one - it's probably already shouldered.
The amount of power transmitted by a mini accessory belt is really very small indeed and well within it's working range.
