Both are just approximations, but you know of course that flashing red cannot mean 0%. If it were 0%, the light would not illuminate at all.
I've been using Di2 for ten years. Here's how my experience tells me to take them . . .
Solid green - 75%+
Flashing green - 50% to 75%
Solid red - 15% to 50%
Flashing red - Less than that.
As a practical matter, I recharge when the green starts flashing. Even though I put about 8,000 miles per year on the bike, I still only have to charge the battery three or four times a year. Charging on flashing green means I never have to worry about it.