use the proper tool and follow the directions in the service manual ( you do have one, right) you will get alot of miles out of your belt drive system if you do this. it's a very touchy adjustment if you want it correct.
Working in an indy shop that only services harleys, I see many bikes come in with the belt tightened so that belt feels more like a guitar string. We get very few broken belt jobs but we get many bikes (2 in there as I write this) with trans main shaft seal leaking because the belt was run too tight for too long, the worst case being a customer ignoring the leak until he smoked his trans. I would rather err to a little looser than spec.
This is with fingers. Works fine. Hardly any stretch in these belts anyway. The Harley gauge is a worthless overpriced piece of junk.
Glad you came along to tell me that the tool I've used for the past 25 years to set my belt tension is "worthless, overpriced junk". Whatever would I do without you?