Lewis & Short

sācōma, ătis, n., = σήκωμα, a counterpoise, a weight precisely balancing something in the opposite scale: pendet ex altera parte aequo pondere phelli sacoma saburrale, Vitr. 9, 8, 8.
Hence, ad sacoma appendĕre, precisely, exactly, Vitr. 9, praef. § 9.