The water volume used or needed when applying Eraser A/P or other herbicides, depends on the density of the weeds that are being sprayed, and the type of equipment that is being used. Most customers do not have the type of equipment that can provide adequate low volume coverage like is indicated in the 3 gallon per acre rate. On average, most will use between 20-40 gallons of water per acre, sometimes more, to make sure you get sufficient coverage over the target weeds when broadcasting the acreage. The standard application volume for post emergent treatments is a gallon of water to every 1000 sq/ft you intend to spray over.
While a non selective herbicide which in theory should kill wild onions, Eraser A/P is not labeled to target wild onions. You may consider using Ferti-lome Weed Free Zone, which is a post-emergent herbicide that controls a wide range of broadleaf weeds in cool and warm-season residential lawns, including wild onion. Please refer to the product label to make sure your lawn type is included on the tolerance list, and for application instructions.