private double trafficShiftHeadroom(Instant now) {
        if ( ! nodeRepository.zone().environment().isProduction()) return 1;
        if (canRescaleWithinBcpDeadline()) return 1;
        double trafficShiftHeadroom;
        if (application.status().maxReadShare() == 0) // No traffic fraction data
            trafficShiftHeadroom = 2.0; // assume we currently get half of the max possible share of traffic
        else if (application.status().currentReadShare() == 0)
            trafficShiftHeadroom = 1/application.status().maxReadShare();
        else
            trafficShiftHeadroom = application.status().maxReadShare() / application.status().currentReadShare();
        return adjustByConfidence(Math.min(trafficShiftHeadroom, 1/application.status().maxReadShare()), now);
    }
Returns the headroom is needed to handle sudden arrival of additional traffic due to another zone going down
as a multiple of current resources.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/autoscale/ClusterModel.java