Methods and apparatus for adjusting model threshold levels are disclosed. An example apparatus includes a volume estimator to determine volume estimates corresponding to a total number of users that satisfy a model threshold within a first period of time; an estimate adjuster to when a ratio of (A) modeled users that satisfy the model threshold to (B) a sum of the modeled users that satisfy the model threshold and deterministic client device users does not satisfy a minimum ratio threshold, increase the number of modeled users to satisfy the model threshold; when the total users that satisfy the model threshold does not satisfy a total volume target, increase the number of modeled users to satisfy the total volume target; and updating the total volume target based on the increased number of modeled users; an adjustment factor generator to apply an adjustment factor based on the volume estimates and the updated total volume target to an absolute count of users to generate an adjusted count of users; and a model threshold adjuster to adjust the model threshold based on the adjusted count of users.