BM told that "BiShares is more secure after 1 block than Bitcoin is after 6 blocks and more secure after 51 blocks than Bitcoin after 120 blocks."

How is that actually calculated?

Great post delulo

99.9% of the time with a solid slate of delegates and a block that we received "on time" you never have ANY forks. 100% of the time we have had over 60% delegate participation and 99.9% of the time it has been over 90% delegate participation. So if your transaction makes it into a block on the public chain the probability of you being on a minority fork is 0 and your client will be YELLOW or RED to warn you if there are any problems.

After 51 delegates you have "majority approval" which is effectively a checkpoint because no delegate is going to "double sign". 510 seconds, 8.5 minutes. Bitcoin effectively declares 120 blocks (20 hours) to be a checkpoint (and thus allow mining rewards to be spent). 8.5 minutes vs 1200 minutes.