[IFoldPktOnes]

Short answer:
~48% minimum, positional disadvantage not a big factor in this case.

Crazy Theoretical Ranting:
Just because you are at a positional disadvantage doesn't mean you should take marginal -EV (by ICM). What determines the extent of -EV it is correct to take is how effective your action is at reducing your positional disadvantage. A 0 EV action reduces your positional disadvantage by increasing the variance of your expected stack size in such a way that leads to a more favorable situations in terms of stack sizes. The favorable situation in this case would be having the t1800 stack covered, hence reducing your positional disadvantage vs that opponent.

If you call SBs all in and double up you cripple his stack to t400 and the next hand you will still be at significant positional disadvantage (think about why). IMO the reduction of positional disadvantage isn't very significant in this case.

Also, in general it would be better to take marginal pushes than marginal calls to reduce positional disadvantage. This is because calling outcomes (eg. 50%-win/50%-lose) lead to more symmetric variance about your expected stack size, compared to the outcomes when you push (eg. 70%-steal/20%-lose/10%-win) which leads to asymmetrically favorable variance about your expected stack size.