I think I would need more information:

1. How big is X in relation to the blinds. If X=SB - then 3 to 1 is probably too high (because the "big" stack here is still inconsequential to the rest of the table and to the blinds - playing any hand is all in). In this case the closer to 1 to 1 the fairer you would get in my opinion. On the other side of the equation, as X becomes larger and larger (say 50+ BBs), the skill level becomes more and more important. Since you've already stated that the players are equally skilled - this seems like the case where 3 to 1 is the better route.

2. On another side of the coin is the value of X vs 3x. It could only take 2 hands for the situation to go from X to 3x all the way to 4x to nothing. Although that situation is unlikely barring some cooler type hands - it is still a factor to consider. After one double up - the big stack has just evened out entirely.

3. Position on the table and the other players around. If the short stack is followed by a couple of weak players - it would stand to reason he'd be able to increase his stack relatively easily; by contrast, if the shortie is followed by the big stack - that decreases his odds as well. Stronger other players in position related to the others is a pretty big factor.

All of that said - I would probably say smaller stakes are more reasonable here.because the variables involved are relatively unknown and unpredictable. In this particular case, as an observer, I would think 2 to 1 is a good and solid agreement; as the short stack - I would be happy to take 3 to 1

how many runners are left? the answer is better than 3:1 if there are more than 2 and is a function of the number of chips left in play. The answer approaches 1:1 as the number of runners approaches infinity.

what do you mean by "20k runners" I don't know this slang. If you are talking about the field size, I don't get how it matters if there are 20k or 500.

huh?

How is player A's stack vs the average?

If it's below the average, then it's more like 5:1 or more...having more chips gives you so much more power.

I think there's a number of factors.

what are the blinds and the stack in relation to the blinds?

lets say this is the sunday 215. You start with 10k chips @ 25/50

on the first hand player A folds, but player B runs his AA into KK and QQ preflop, and now has 30k in chips.

he is NOT a 3:1 favorite at this point to outlast player A. However, lets say this is the final table. and both have 1mil in chips. Player A folds and player B wins AA vs KK and QQ for 3mil. lets say blinds are 50k/100k. now that 3x advantage is much more important, and his chances of lasting longer are much greater than 3:1, IMO.

I won't bother trying to come up with an answer on what would be fair, simply because I think it depends so much on the size of the stacks in relation to the blinds and stage of the tourney (people play far better near the end than at the start, even in a tourney as weak as the $215, or even in $1mtt's. )