It's not 25pc top up, it's 75pc miles 25pc cash.
For example, an award requires 20k miles plus taxes. When redeeming online you can use the slider on the final page to bring it down to 15k miles plus a cash component plus taxes.
The rate per mile varies greatly depending on the class and route. I've paid around 350USD equiv for 22.5k miles removed for AUH-SYD in F, and about 130USD for ~3k miles removed for AUH-BEY in J.
You cannot purchase miles with EY.