...

#> show configuration chassis
cluster {
   reth-count 2;
   preempt {
      delay 360;
   }
   gratuitous-arp-count 4;
   redundancy-group 0 {
      node 0 priority 100;
      node 1 priority 1;
   }
   redundancy-group 1 {
      node 0 priority 100;
      node 1 priority 1;
      preempt;
      interface-monitor {
         xe-0/0/19 weight 255;
         xe-7/0/19 weight 255;
      }
      ip-monitoring {
         global-weight 255;
         global-threshold 200;
         family {
            inet {
               192.168.200.2 {
                  weight 100;
                  interface reth0.510 {
                     secondary-ip-address 192.168.200.4;
                  }
               }
               192.168.200.3 {
                  weight 100;
                  interface reth0.510 {
                     secondary-ip-address 192.168.200.4;
                  }
               }
            }
         }
      }
   }
}

ip-monitor should monitor outside/internet link, both addresses of the Arista switches. The secondary-ip-address is the IP that the secondary node in the SRX cluster will use to ping, it should be a free IP in the linknet (in this case .1 is srx, .2 and .3 is arista, and .4 is used on the srx secondary node)

#> show configuration interfaces reth0
   description "To dist-L3";
   flexible-vlan-tagging;
   mtu 9192;
   gratuitous-arp-reply;
   redundant-ether-options {
      redundancy-group 1;
      minimum-links 1;
   }
unit 1 {
   description "description";
   vlan-id 1;
   family inet {
      mtu 1600;
      address 10.20.50.6/29;
   }
}

...