How do you properly calculate the flying speed given by the Eagle Whistle when you have speed bonuses?

Question

Using the Eagle Whistle...

Wondrous Item, Rare

While you blow an eagle whistle continuously, you can fly twice as fast as your walking speed. You can blow the whistle continuously for a number of rounds equal to 5 + five times your Constitution modifier (minimum of 1 round) or until you talk, hold your breath, or start suffocating. A use of the whistle also ends if you land. If you are aloft when you stop blowing the whistle, you fall. The whistle has three uses. It regains expended uses daily at dawn.

Tales From the Yawning Portal p228

How do you calculate the speed if you already have bonuses to your speed?

The problem here is that two speed bonuses exist : those that apply only to your walking speed (such as the Elk Barbarian Rage or the Bladesinging from Sword Coast Adventure Guide) and those that apply to all your speeds (such as the Mobile feat or Barbarian Fast Movement speed increase).

My question is how to calculate the flying speed given by the Eagle Whistle when you have walking and all-around speed bonuses :

A) [ base walking speed + walking speed increases + all-around speed increases ] x 2

B) [ [ base walking speed + walking speed increases ] x 2 ] + all-around speed increases

For example, would a Bladesinging Mobile Wood Elf using the Whistle have a flying speed of 110 ( 35(elf) + 10(singer) + 10(mobile) * 2) or 100 (35(elf) + 10(singing) * 2 +10(mobile) ?

Answer 1

It is calculated on the walking speed

The Eagle Whistle states:

While you blow an eagle whistle continuously, you can fly twice as fast as your walking speed

You would make your calculation on what your Walking speed is. If you have other bonuses or increases to your walking speed, those come into play first - because they effectively change your walking speed.

In your specific case of the Bladesinger Mobile Wood Elf, you would:

1. Calculate your walking speed (base elf 35 + 10 mobile + 10 from bladesong) to get your characters Walking Speed of 55.
2. Flying speed while blowing the whistle would then double that walking speed for a total of 110.

The key here is in the calculation of your walking speed. The Whistle doesn't state to only use the racial walking speed, so it therefore calculates based on your character's walking speed which includes all relevant bonuses.

Beware that it could change!

In your case, should your bladesong run out, the calculation would change as well and reduce your flying speed to 90.