Weight of 255 should trigger failover.
If there are 3 interfaces in reth, total aggregate bandwidth available is 3Gb. If bandwidth goes below 1.5Gb then it should trigger failover, meaning if 2 out of 3 interfaces was to fail then it should failover, so i'd set my monitoring weight to 128 each child interface for that RG group.
3Gb = 3 Links
2.5Gb = 3 Links
1.5Gb = 2 Links
1Gb = 1 Link
Hope this helps
Bilal