what does "rule of thumb" mean

Question

 The table displays the values of the loss functions for the five VaR measures over three indices. The smaller the loss function, the better the VaR

measure. We compare the loss function across models by computing the difference between the loss functions provided by the model in the column and

that in the row. A block bootstrap is performed in order to test for the statistical significance of the loss function difference. The block length is chosen

based on the following rule of thumb: (number of observations)1/3. We rely on 1000 bootstraps.

Answer 1

همچنین : حساب تخمینی / حساب سرانگشتی

Answer 2

یعنی: قاعده کلی