There is a chart at:

How to Apply Sulfur to Increase Acidity

that gives the amount in lbs/100 sq.ft.

It would be .35lbs for 10 sq. ft.

I would not apply it all at once. I would apply 1/4 now and 1/4 about every 3 months: (June, Sept., Dec., and when the ground thaws next spring.)

The reason for this is that it takes a while for it to get down to the depth, so this would keep from getting too strong at the surface.

Of course this needs to be applied under the mulch. Just rake the mulch back and then sprinkle it back in place.

Here is a link that might be useful: How to grow rhododendrons and azaleas.