Re: Donny Gruendler here!
In regards to playing fills within your time -feel -- that can be quite difficult (even for the most experienced players). With this in mind, I share this exercise with all my students at PIT:
At a comfortable tempo on your click track--- Play one bar of time followed by a one bar fill. Thus, you are playing a two bar phrase. (If you use an 4 or 8 bar phrase and you are a "little off" on your transition -- you still have many bars to get back on the click.)
However, my suggested method does not allow you time to adjust to the click. (as your fill transitions into only one bar of time). Therefore, this will help you to work on (and tighten up) your transitions in a much more focused and effective manner.
I hope this helps.