Pilou, it is not the same thing. When you use the transform>scale (123D) the dialogue box shows the amount of scaling only (and that by perceiving the beginning of the scale as '1' unit), you can not put there a value to transform the whole object, automatically, to that value's scale.

Imagine that we have a circle of 15 mm diameter and we want to make it 11.5 mm... we can not write to the dialogue box a simple '11.5' so that by clicking 'ok' the circle may acquire a diameter of 11.5 mm. You can not make even a subtraction -by your mind- and put it to the box (in that case 3.5 mm), because, as I said the app perceives the beginning point of scaling process as '1' unit. And that, of course, makes things difficult to calculate.

The needed thing is just having a measuring tool (as is the 'custom distance' add-on now), measure the distance you want to change, alter the appearing value in the box, click 'ok' and acquire the object with the changed dimension. So simple!

As you understand, the dimensioning is extremely important when you deal with prototyping for objects to be manufactured (as in the case of jewelry).