Out of Hours Call Centre Contracts- SLA's

Good Morning All, Interested in hearing your experiences on outsourced call centre contracts. Currently looking at renewing a call centre contract and with it implementing a number of SLA's that target the following area's: -Abandonment rate -Maximum wait time -Average wait time -MI report submission To maximise value for money I'm looking to impose financial penalties where a provider goes below/above a pre-determined threshold. The penalties will be a percentage discount which would be applied to the following months invoice balance. The penalty amount would start from 1% and go all the way up to 10% for each measured area. Would appreciate any views and successful experiences you've had with SLA penalties and what levels of penalty you've used. Regards Josh Chiplin