usable screen area in landscape mode

Hi there,

I am trying to figure out the usable space on Apple mobiles.

The iPhone 5 has a definition of 1136 x 640 pixels.

I found those values about the height Safari uses : status bar 20 px + address bar 60 px + buttons bar 44 px. Total : 124 px.

So I infer that on an iPhone 5 the usable screen area in landscape mode is 1136 x 516 px.

For the iPad 2, whose definition is 1024 x 768, the same reasoning would give a usable screen area in landscape mode of 1024 x 644 px.

Is this correct ? Any suggestions from an experienced responsive webdesigner are appreciated. Thanks.

Gil