While you can spend additional money on some sort
of hackle gauge, you really dont have to.
Or for measuring the length of the wing on streamers.
Since you already bougt the hackles and hooks, use them to your advantage.
Splay out the hackle you intend to use on your fly and measure it against the hook shank.
From the eye of the hook to the bend of the hook shank (the straight part of the shank before it begins to bend) is equal to 1 1/2 times the hook gape. Perfect for hackle length.
Whatever method you use is totally up to you.