buy 1 or 2
I would say: on the contrary: with that price buy one only if you can share the expense with at least two other users. Otherwise make your own. Search in this forum for homemade manometer, read it, build it and have the satisfaction you've made something good for a couple of bucks.
You can also benchsync. I can sync my carbs within 2 cm Hg by moving an allen key under the slides and 2 cm Hg is well within specs. But first of all: ask yourself if your carbs need to be synced at all. Except for the first 750's that had cables, all Fours have a cam that operates the carbs simultanously. Unless the PO messed with the adjusting screws (how likely is that?), there's no need to suspect your carbs to be out of sync.
