Are you trying to establish a relationship or ratio between the fluid weight of front and center diff? I honestly don't think there is one. If anything there is a relationship or ratio between the front and rear. The rear is usually somewhere around 50-75% the weight of the front in most cases, whether nitro or electric.

IMO Center diff (CD) fluid weight is totally dependent on the amount of power, traction and amount of weight transfer under acceleration. With more power you need heavier fluid. I've had two 8ight-t 2.0 truggies (should've never sold first one) and the first one had two different setups. With a Castle 1515 2200kv on 4s my CD was ok with 50k but when I went with a much more powerful Castle 1520 1600kv on 6s I was burning up the front tires with 50k and needed 100k.

Diff fluid weight depends on a lot of factors. The physical size of the diff and the spider gear count (Traxxas Revo/Maxx diffs need much heavier fluid than 1/8), amount of power and traction as well as driver preference. I personally like a looser cd to retain some diff action as it helps turn in and is more predictable powering out. I would think with 5k center you have enough diff action. The only reason to go thicker is if the front tires are ballooning or you'd like to experiment and find better handling. If you're going to experiment I'd pickup some 20k, you can always mix in some of your 5k to lighten it up if 20k is too thick.