In all honesty, I don't mess around with patched phrase at all.
I know the video you are talking about. If I remember correctly the dude just took a loop that had a steady tempo and he set up slice points that were equidistant from each other.
I've tried it out in the past with samples that weren't dead on and I do find it difficult to impossible to get it right. I think alot of it has to do with tweaking the slice/marker points correctly so they are in the right spots.
My understanding of patched phrase is that it is similar to warping samples in software. So you have to set warp markers and each little slice gets adjusted to the desired tempo (according to the algorithm that the MPC is using to adjust the little pieces).
So having the warp markers in the right spots (say on every quarter note of a 1 bar loop) and not necessarily equidistant (especially when you are dealing with a sample with tempo changes) will yield a better result. Not necessarily a result that sounds exactly like the original sample but something that will be able to lock to the tempo of your project correctly.
Someone correct me if I'm wrong here please, since I'm actually also kind of confused as to how patched phrase works. (Sorry if my explanation sounded kind of confusing)