If I understand you correctly, I think the anticipation of hitting the fill button is different for each brand of keyboard. In general, Korg is a bit more sensitive than others, but can easily be learned. Yamaha seems more forgiving, allowing the user to hit the button close to any of the beats in a measure. Many will trigger a fill on beat 3, or 4 of a 4/4 measure, or beat 2, or 3 of a 3/4 measure. Sometimes, there's a sweet-spot for the hit, but mostly, you can punch the button anywhere in the measure you want the fill to live in.

Is this what you're asking? Did I make any sense?