Generate a Random Number between 0.0001 and 0.002 in Objective C (iOS)?

Does anyone know how I could generate a random number in a range in iOS? I am currently working on a synthesizer in iOS (using SpriteKit and AudioKit) and I am trying to modify the loudness of the synth with changing its variability whenever a slider is being moved as well.

This is what my code looks like:

[Synth setAmplitude: 0.5 + (slider.currentValue * loudnessVar)];

where 0.5 is the default amplitude value and loudnessVar is a random number.

Since, the slider returns values from -170 to 170 , I would need a relatively low number in order to set a value between 0 and 1.

Is anyone able to help with this?