2 Numbers To Remember Keep In Mind With Faders.
When mixing a boom and a radio
I've found that when I am on a wider mid shot or wider than it's necessary to do a combination of mixing in the radios with the boom. This gives me still a sense of space with a little boost or a punch with the dialogue from the radio microphone.
Now you don't want both faders up because that will give phase issues so I generally have one fader up and then the other at 20% of full. Depending on how wide the shot is you would have either the radio or boom as your main.
2 Numbers To Keep In Mind For Levelling
Slighly louder or quieter would be around 3dB and doubling or halving the perceived loudness is around 9/10dB.
On my mixer I can see a digital readout for where the fader is at so usually keep clocking up in 3dB levels.
Now you can't just be staring at individual read outs because you have to get used to your headphone levels and trust your ears.
If you have a digital readout then try to get a feel for where it is on your mixer. This is obviously after setting your gain properly.
Of course let's me know your thoughts below.