I am not an expert on this stuff, but I think you need to figure out the standard error for your sample. Once you know standard error, you can use it to judge how close your estimate for the mean should be and with what confidence level. Essentially, standard error is the standard deviation of your sample. So, if you were to find the mean mass of every possible sample (from 1 block, all the way up to 600 blocks), you should be able to calculate the approximate standard deviation of the set of samples based on the actual standard deviation of the masses and the size of your sample. So, using the binomial theorem, since your sample is greater than 30 blocks, you can use the formula that standard error is standard deviation over the square root of your sample size.
Where is your standard deviation of 0.248Mg=248kg, and your sample size . Need any more assistance?