Hello Carl,
your understanding is correct you actually need to have a difference of less of 10 to use normal tracking decrement
primary router
int fas0/0
ip address 10.10.10.2 255.255.255.0
standby 10 ip 10.10.10.1
stannby 10 priority 105
standby 10 track ser0/0
standby 10 preempt
secondary router
int fas0/0
ip address 10.10.10.3 255.255.255.0
standby 10 ip 10.10.10.1
standby 10 preempt
the key command is the last one that provides capability for secondary to take role of active HSRP on failure.
Hope to help
Giuseppe