"safe" overclocking presets, values copied from raspi-config script.

 

none	arm_freq=700 core_freq=250 sdram_freq=400 over_voltage=0
modest	arm_freq=800 core_freq=300 sdram_freq=400 over_voltage=0
medium	arm_freq=900 core_freq=333 sdram_freq=450 over_voltage=2
high	arm_freq=950 core_freq=450 sdram_freq=450 over_voltage=6
turbo	arm_freq=1000 core_freq=500 sdram_freq=500 over_voltage=6

source: https://github.com/asb/raspi-config