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.
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) ?