KDP has the right answer, but we need to know why it is the right answer. Red must move to the edge of the prime and take his chance there. Why? The answer is, "If not now, when?" He has to do this. White's answer to this is clearly going to be that he will attack that checker. Red will have to hope to hit back. If and when he hits back, he needs a four point board, not a three point board with a blot, so clearly 6/3(2) is mandatory. That only leaves 13/10 as a viable three. Why do we not mind leaving a blot on the 10pt? Because White isn't going to hit it, unless he rolls 3-3. This 1/36 chance of what is in any case a very strong roll for White is a price worth paying to make the three point and a bonus, it gives White a safe playable 6 next turn.