Need advice on Calculating NDWI

Gillian

Hello. I'm using Planet.com 4 band data (RGBI) and need to calulate NDWI. I'm finding that my results with the NDWI shader are just grey, and all other shaders look very odd - a key loch in the terrain should be blue!. None of the permutations I've tried look anything like the Sentinel NDWI from about the same time period (which looks great). I'm using the following equation ((B2 * 9.086433827952812e-05) - (B4 * 0.00017310169009729403)) / ((B2 * 9.086433827952812e-05) + (B4 * 0.00017310169009729403)) - the multipliers are there to convert radiance values to reflectance values.

Any afvice on how to set the Raster calulator up would b emuch appreciated!