How can I use a Variable as Value inside of a timeline?

Player Movement Speed
First i am checking if the velocity is my crouch speed
Then i do have a timeline, wich goes from 1400(200 more than runspeed) to 300(crouch speed) and i am setting the MaxWalkSpeed to that value and then add a forward movement input as long as i hold Left Ctrl.
Everything works fine, but it does it with 1400 when i am running, walking or wall running(yes, i do have that, but inspired by a video).