How do I measure the Organic Uplift/Organic Multiplier for retail apps ?

The most straight forward method woul be:

Organic uplift = (All installs – Organic baseline – Paid Marketing installs) / Paid Marketing installs

Try to use a large dataset of paid vs organic installs, slice data by OS & country and remove outliers

If you have data of your own, you can just scatter plot daily paid installs vs daily organic installs. The slope will be one way to quantify the organic multiplier paid acquisition can have on your organics. From our own data (with games) we observe inflection points as daily tracked paid installs reaches a certain level. Presumably this is because the games have reached some level of visibility in the top charts that drives more organic downloads. I'd imagine this is varies for different parts of the app store, but should give you a directional process to follow.