How do I stop FCPX retiming a generator - Tried Markers

As the title suggests - I've created a generator (screen wipe/text/spinning asset) the longest I need it to be is 40 seconds, the shortest is about 10.

When I shorten it in FCPx it makes the whole thing quicker. I read another post on here about markers so tried a "Build In Mandatory" - to do this I:

Set my playhead on the start of the project - added a *** marker, set the in to 00:00 and duration is greyed out at 00:00 this put a yellow dotted line over the entire project.

Saved it as a generator and opened it in FCP

However this made absolutely no difference when shortening the clip. What am I doing wrong?

The desired outcome is i can make the clip 10sec and it still acts the exact same at the start as the 40 second one.

(I know I can use a compound clip but I've got about 40 of these over multiple projects so this will get very messy)

Thanks,

Greg