15% of a 500 calorie diet is a lot different from 15% of a 2000 calorie
diet or 15% of a 4000 calorie diet.

A) If carbs were "poison", they would be expressed as absolute weight,
not relative measure, such as percent.

B) Some "low carb" approaches let you find your limit of "low carb" by
initial restriction and gradually adding carbs back. So, "low carb" is
specifically tailored to your body response.

C) Some "low carb" approaches use different percentages.

D) Some "low carb" approaches use actual carb weights.

E) Some "low carb" approaches use instead a subset of all carbs, based
on some body response such as Glycemic Index, which of course varies
greatly between people for the same foods.

F) All of the above.

G) None of the above.

Some days, I vote for both F)  and G).

I think the absolute amount of carb is more important than the
percentage.  Your body reacts to the amount of glucose in the blood,
which isn't based on percentage so much as amount.

Atkins says start at 20g carb and up it gradually each week until you
find your own tolerance based on weight loss.

Eades say 30g carb until health issues resolve, then up.

Bernstein says 30g carb forever and ever.

My blood glucose meter says there's no difference between 30g carb and
50g carb.

Most people would likely greatly improve their diets just by quitting
white sugar, white flour, white rice and potatoes and replacing those
foods with nonstarchy vegetables and low-sugar fruit.

Davis says go ddown to under 50g carb if you have weight to lose,
otherwise just cut out the wheat products and sugar.

How low you go depends on why you're doing it.