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The allen key method is extremely subjective .. hence the various mechanical or audio/electronic attempts to standardise the procedure.
But I've done maybe ten belt changes on 750/600 engines and always use allen keys.
Once you get the feel of it, the confidence increases.
I work on the principle that a little slack is probably fine, a bit tight could be disasterous.
I only ever move the adjusters with light hand pressure (as you describe) which is more than adequate. I would never even consider using leverage. in fact its easy to overtighten using just hands.
My preferred technique to measure the tension is to push the belt away from the pulley using two fingers, one either side of the contact point, and then test the gap with the allen keys (rather than force the key through on its own and try to judge how much effort I'm using). Then I aim for what you might call a "feeler gauge fit" with the smaller key and "elastic interference" with the larger.
Using this technique I find that I can easily discern whether I am holding both the outer surface of the belt and also the allen key accurately parallel to the pulley axis. This is not possible if the allen key is simply forced through the gap in the conventional way and this is one of the factors which give me confidence in the technique.
Hope that makes sense .. particularly the last bit. Its hard to put into words .. hence my late reply.
Its worth repeating that I aim to achieve an acceptably minimal tension while being absolutely certain to avoid overtensioning.
As a kind of double check, I also try to twist the belts in their longest run after tensioning them and aim to be able to twist them around 45 deg. However this again is extremely subjective.
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