Sorry, but I cannot help you right now with sizing the balancing disc, but I can comment on the sand issue.
A balancing disc or drum works by providing a "tretcherous path" (pardon my spelling) for the water to flow. For it to properly work, you need *tight* clearances. For this reason ALONE, you should NOT use such a balancing device when you have a solid-containing fluid. In other words, do not use a balancing disc or drum.
Regards,
Tim