Morning
@Jac123
I use the old mechanic’s technique of 3/8 inch (approx) deflection over the long gap, or less then 90 degree twist of the belt on the long gap.
Of note most dust issues come from out of alignment of the pulleys or improper alternator install. Use a straight edge to check the alignment. Be sure the through bolt on the alternator is the right size and the alternator does not wobble. You may need a spacer or shims/sleeve to get this right. Be sure the bolts are all properly tightened. It also helps to buy a good quality belt.
@mainesail had advise on alternator installation where he identified these important elements.
“It is up to the installer to account for:
Correct pulley to belt alignment
Proper pivot bolt diameter
Adequate belt adjustment range
Correct bolt diameters, threads and lengths
Proper torques and thread depth engagement
Locking washers or nyloc nuts
Correct belt & length
Adjuster arm fitment, length or spacers”
There will be a little stretching of the belt. Not a lot. Proper install of alternator and you should get good service from your belt.