I think you'd have to be careful with the RRF as light plays into the colour. I've had it growing packed densly (so much it didn't move) in a tank with the light about 1" above the surface and there was a very noticable line across the plant growth showing the extent of the light - red under the light and green around the edges. I've also taken them out of the tank, and floated them in tank water on the window sill and you get the deep red colouration. It a very cool plant but I think the light reaction adds an extra variable that wouldn't be helpful for the dcukweed index.
When it's happy it will grow over the top of itself and almost convert to emersed type branches where only the base sets of leaves touch the water and root with the offshoots growing up so it's packed an inch or more thick and out of the water supported by other plants below. On the other hand, if it's not happy it disintergrates readily.