How to write a MQL query to setup the alert policy if node uptime is less than 1 with 20 % nodes

Question

sample MQL query :

fetch gce_instance | metric 'compute.googleapis.com/instance/uptime' | filter (metadata.user_labels.application_family == 'sample') | align rate(1m) | group_by [metric.instance_name], [value_uptime_mean: mean(value.uptime)] | every 1m | condition val() < 1

here unable to add the condition if 20 % nodes affected out of total nodes.

help us to write a MQL query if condition value zero with number of nodes are affected more than 20 %