1 amp is fine. You can calculate charge rate using the capacity of the battery. Your charger charges in Amps. Batteries are rated in milliAmps, so you gotta do some conversions. 1 milliamp is .001 amp. So you just move the decimal 3 places and that give you your charge amperage.

Example:

1400 = 1.4A
1100 = 1.1A
and so on...

You can charge packs at higher amperage, but the performance boost by doing so isn't necessary for a receiver pack and is also hard on the batteries themselves.