"Animate In" timing - title templates for Final Cut X

Using: Motion 5 to create title templates (lower thirds) for Final Cut X

Issue I am running into is the length of the title.

Motion project is set to 20 seconds

Title (lower third) has a basic animation (fade and rotate) in that created in motion. When I use in Final Cut X everything works great, if I want the lower their to stay in for exactly 20 seconds. If I shorten or lengthen the title, the animation responds by animating very quickly or slowly.

Any suggestions as to how I can make my custom animation be an "animate in' option in Final Cut, so I can use the lower third at any length?

Thanks so much for your time and input Mark! That is exactly what I was looking for.

I added a marker, then "edit marker" and make it a "build-in optional". Perfect.

Thanks again!