[Galwegian]

Here is the general formula. I used the rule of 4 approximation for calculating equity on the flop.

If
P = pot size on the flop
S = stack size
B = opponents bet size
n = number of outs that you have

let c = ((25-2n)S-P)/((25-2n)S+(25-n)P+25B)

If your opponent will fold to an all in reraise more than c of the time then the all in reraise is better then folding. You can easily rewrite this in terms of spr, as long as you replace bet size by bet-to-pot ratio. Of course this formula is beyond most people's real time arithmetic skills (certainly is for me anyway).

Something that is interesting to note about these formula is that the amount of fold equity required gets smaller as the size of the all in reraise gets smaller. This seems a bit counterintuitive at first, until you observe that for small all in reraises your opponent will be getting good pot odds to call, so the amount of FE that you actually have gets smaller as your stack gets smaller. To find the right time to use this play you must balance the two things. Your stack must be big to get FE but also must be small enough to ensure that you don't need too much fold equity for the all in to be +ev. The formula in this thread suggest that an spr of between 3 and 7 (spr above will often mean that the villain will have to fold more than 50% of the time) is the optimal range for this all in reraise play to succeed. Of course this depends very much on the precise situation.