Here is what I run mine with (NAA renderer), very much underclocked. This is from config.txt
#Serious underclocking
force_turbo=1
arm_freq=600
arm_freq_min=600
gpu_freq=150
core_freq=200
core_freq_min=200
h264_freq=200
isp_freq=200
v3d_freq=200
sdram_freq=300
sdram_freq_min=300
avoid_pwm_pll=1
over_voltage=-8
over_voltage_min=-8
over_voltage_sdram=-8
over_voltage_sdram_c=-8
over_voltage_sdram_i=-8
over_voltage_sdram_p=-8
#Disable HDMI, WiFi and BT
hdmi_blanking=2
dtoverlay=disable-wifi
dtoverlay=disable-bt
#Disable LED
dtoverlay=act-led
dtparam=act_led_trigger=none
dtparam=act_led_activelow=off
dtparam=pwr_led_trigger=none
dtparam=pwr_led_activelow=off
#Turn off ethernet port LEDs
dtparam=eth_led0=4
dtparam=eth_led1=4