Storyline Timeline - Levels of Granularity/Minimum Timeslice

Hi,

I have an animation that I want to be 1 x 8th of a second in length, but no matter what I do in terms of dragging it there, it always reverts to 1 x quarter second, as per image below.

Why? Is there some minimum setting or something, so that you can for example seperate things by 1 x 8th second intervals, but you cannot make them this short?

Thanks.

Bruce