In ordinary conditions it’s reasonable to expect broad areas of weak sink between evenly spaced thermals. The old Soaring Flight Manual proposed a sink rate approximately one fifth of the average lift. (For example, if thermals are averaging 500 fpm, the inter-thermal sink rate would be about 100 fpm.)
However, in an article for SOARING Magazine, Dick Johnson’s data from a series of test flights indicated ten percent, or 50 fpm sink between 500 fpm thermals. This might seem a large disparity, but look closer. Johnson acquired his test data by flying a sailplane at one speed only, and the results measure the combined sink of the air the glider at that speed.
Of course we should always fly faster than best L/D speed in sink to maximize glide, which further increases the total rate of descent. And if we make that adjustment, our total sink rate will increase significantly, closer to what the Soaring Flight Manual suggests. According to this logic, inter-thermal speed should always be faster than best L/D speed — and still more if we expect strong lift ahead.