Guidelines or formulas for how much water to dump in narrow or weakthermal racing?

Hi, all.

When racing a typical 1-2 hour flatland race in, say, a Diana2 or Ventus3-15 (15 meter ships with water ballast and flaps), are there any guidelines or formulas for how much water to dump, particularly when thermals are weak or narrow?

Here are the parameters I'm thinking would apply (but I'm no expert yet- thus the question):

1. Average thermal strength
2. Average thermal width
3. Average distance between good thermals
4. Height band for the day

I was hoping to find something on this subject in a glider racing book or the Diana2 flight manual, but no luck so far finding either googling.

Ben Ethridge