I haven't got one myself, I assume your looking at buying some used forks with a cut down steerer? if you measure from the base of the headtube to the top of your top cap that should give you the maximum length you'd need within 10mm or so. Hope this helps.
According to the Spesh site your headtube lenght is 95mm so you'd need to add on to that the external cup if it has one below the headtube if it has one plus the height of all of the spacers, stem and top bearing cover.
As above, headtube is 95mm long, typical stem stack height is around 45mm, handful of spacers for adjustment plus a bit spare for comfort - I'd say you're looking for 170mm minimum but you'll only know for certain by measuring what you've got. Just measure from top of fork crown to top of stem cap and you'll have your (approximate but good) answer. For instance, your stem might be 50mm high, not 45mm etc.
All I would say is that if the fork you're looking at is less than 170mm then you'll definitely be pushing your luck without measuring your current set up!!
Just do as shown in the attached pic but to the top of your current steerer (pic shows an un-cut steerer) and always err on the side of caution. If in doubt, assume it needs to be longer - too short renders the fork useless (without spending money on getting a new steerer fitted!) and you can always trim down later.