After replacing the alternator, its belt and the A/C idler+tensioner pulley
on my car I used the old parts plus a digital crane scale and turnbuckle to
set up a variable belt tension fixture to check my Gates Krikit 91107 and
91132 tension gauges.
https://www.amazon.com/gp/product/B000MUTAGS/ref=ask_ql_qh_dp_hza

The 91107 is rated for 3 and 4 rib serpentine belts as well as traditional V
belts, though the ad copy doesn't make that clear.
https://www.gatestechzone.com/en/tools/universal/krikit-i-and-krikit-ii-belt-tension-gauges
"Krikit I is suitable for V-belts and narrower multi-ribbed belts (3-4
ribs): 30 to 160 lbs or 15 to 75 kg;"

They are easy to misuse. I found that if the round end doesn't sit flat on
the belt it will read low, and if I push the round end down with a second
finger it may read high. A little drag toward the round end kept it in
contact without excess pressure and gave a reading of 1/2 the pulley axle
pull on the crane scale within half a mark, or 5 pounds.

The Krikit is compact enough to fit the very cramped space of the belt
cluster between the engine and inner wheel well, where I can't see to
measure belt deflection with a straightedge. I had been using the quarter
twist tension check and setting them below spec.

The alternator pivot corroded very solidly from road salt so that I couldn't
increase tension and I thought the occasional squealing was from that,
rather than the harmonic balancer slipping. The first indication I noticed
that it was bad was a wobble in the belts that ran on the outer ring, while
the center hub belt ran true.