Getting Filter Length scales

CFDtoy

Hello

I am performing multiphase LES calculations. My grid cells, hexahedral, have smaller dimensions in x,y as compared to z direction. Lets say x ~ y , z ~ 5x. When i get my filter length scale for SGS viscosity computation, I use length ~ (vol)1/3 and found that it was providing very high viscosity. What is a better way to compute the filter length scale?

If someone can direct me to any references, paper that would be great.

Thanks,

CFDtoy