In the BoB TOD, it was subjective based on how much of the targets the LW destroyed mixed in with how many of them made it home and how many RAF lived through the attack.
Frame 1, the Axis leveled everything, but lost everyone. RAF lost a lot, so it was a slight Axis victory.
Frame 2, the Axis leveled everything, and most everyone. RAF lost a lot again, so it was a moderate Axis victory.
Frame 3, Axis hit London hard, one target pretty well, and one target not at all. Both sides lost equal amounts of aircraft. It was a draw.
It's kinda hard to go off of straight points unless both sides have the exact same strike objectives, and field about the same number of pilots.
Personally, as a CM, I like to visually inspect the targets after the frame to assess damage. Then I count up the percentage of people landing safely, ditching, bailing, captured and killed. So basically I go by who does the best given the objectives I gave to the Frame COs. That's just me tho.
Fortunately, we get "artistic license" when we do our TODs. It doesn't make it consistent, but it keeps it fresh, IMO.