Determining parity or FEC (Forward Error Correction) requirements from percent error
A BER of 25-30% implies that you have only about a 5-10% chance of receiving an error-free byte. This is way beyond the capability of any practical Reed-Solomon based FEC system. You need to look at convolutional coding -- FEC at the bit level -- using something like Viterbi decoding on the receive side.

To answer your original question, the probability of finding exactly k errors in a block of n symbols, given a symbol error rate of p, is:

C(n,k)&times;pk&times;(1-p)n-k

where C(n,k) is the binomial coefficient: C(n,k) = n!/(k!&times;(n-k)!)

To find the probability of having m or fewer errors, you need to add up the individual values for k = 0...m. Then, the probability of having an uncorrectable block (more than m errors) is 1 minus this sum.