@klincecum I do a labor rollup on labor.update, but only on the total quantity of the final operation of the job, so it only runs once per job because we want the wip earned on the job.
The same would work for this material, check conditions pre processing to do the job material check post processing. I would just mark any backflush material complete, but you could do some fancy round to 2 places and check for variances.