I used to use a straight edge to measure off of but don't bother anymore. If you think about it, if you have proper toe (zero to 1/8" out), the skies will center themselves to the chassis so the only use of the straight edge is to set clear vision (bars straight). Which may not be the best way if the steering rod from the steering post to ballcrank isn't easily accessable.
As long as everything is tight (i.e. tie rod ends, ballcrank, etc) I measure toe off of the carbide bolts that protrude through the top of the skies (provided they're not bent) using a slightly taught bungy that doesn't pull the skies too tight together and set the toe at zero.