I've been testing with Fast Fall Uair into Tipper Up smash at around 125-150 depending of the character weight. It seems pretty consistent but on characters like Megaman, Ness, Jigglypuff etc it seems to be much more difficult to land the FF Uair. Alongside that on the floaty side of characters like Ness, Jigglypuff, Mewtwo the set up begins to work at around 125% but at 150% ( Where it works on most of the cast ) they seem to be hit to high for the UpSmash to connect. On most other characters trying to do this setup below ~145% will result in the side of the Upsmash connecting and usually not killing. On the majority of the cast at 150% the FF Uair connects pretty consistently and Upsmash does as well.
* This does not work if Sheik is not facing the opponent, the Uair will not connect