Does improved whip mastery benefit from long arm?

Question

I have built a whip character. The goal of this character is size from enlarge person, increasing reach with long arm, and attacking everything within 45 feet reach using whirlwind attack. I have invested the feats to take improved whip mastery so I can now threaten with the whip equal to natural reach +5 feet. So medium is 10, and large is 15. What I am trying to figure out is if the long arm spell would add to this threatened reach making this threatened space medium 15 and large 20.

Long Arm

Your arms temporarily grow in length, increasing your reach with those limbs by 5 feet.

It says my reach increases, but not my natural reach. Does this distinction matter?

The answer to "how can I threaten with a whip" does not directly answer my question.

Answer 1

There is nothing about a notion of "natural reach" in the rules of long arm. You quoted the whole text. "Natural reach" is probably "reach before taking into account the weapon's reach property" but that's not an official rule, just the one that makes the more sense.

What that text means is that the +5 feet increase only applies to your arms: if you have a bite, gore, wing buffet... attack, it won't benefit from the increase. But the whip definitely will as long as you hold it in your hands.

However I suspect the reach property of weapons doesn't stack with other bonus to reach the way you think. If you stand on square 0, square 1 is next to you, square 2 next to it, etc, a longspear with long arm let you threaten squares 3 and 4. If you are also enlarged you would threaten squares 4, 5 and 6. In your case (whip, whip mastery, long arm) you threaten squares between 8 and 12, or between 11 and 15 if enlarged.