Make Autoscaling scale-out more quickly for customMetric

I'm setting up an Autoscaling group and it's using TargetTrackingScaling that is tracking a custom metric that is being updated every minute (via a scheduled lambda that queries the inservice instances).

It's working fine, but I'd like the scale-out action to happen more quickly. The source of this looks to be due to the ASG Alarm that gets auto created. It's looking for 3 datapoints before triggering, this means it takes 3 minutes for the alarm to fire.

Is there a way I can configure the ASG/Scaling policy such that the alarm only needs 1 datapoint to fire? Or if not can I create a custom alarm and use that instead?

The metric is {Unit: 'Percent', Statistic: 'Average'}