How do you implement failover only?

Don't confuse ratio with priority. Ratio means that the "backup" server will get every 1001th connection in a two server scenario with your numbers.

Consider this example from page 4-13 of the configuration manual:

pool my_pool {
 lb_mode fastest
 min active members 2
 member 10.12.10.7:80 priority 1
 member 10.12.10.8:80 priority 1
 member 10.12.10.9:80 priority 1
 member 10.12.10.4:80 priority 2
 member 10.12.10.5:80 priority 2
 member 10.12.10.6:80 priority 2
 member 10.12.10.1:80 priority 3
 member 10.12.10.2:80 priority 3
 member 10.12.10.3:80 priority 3
}
Figure 4.1 Sample

The configuration shown in Figure 4.1 has three priority groups, 1, 2, and 3.

Connections are first distributed to all pool members with priority 1 (the

highest priority group). If fewer than two priority 1 members are available,

traffic is directed to the priority 2 members as well. If both the priority 1

group and the priority 2 group have fewer than two members available,

traffic is directed to the priority 3 group. The LTM system continuously

monitors the higher priority groups, and each time a higher priority group

once again has the minimum number of available members, the LTM

system again limits traffic to that group.