Does it have to be big? WFF (small amplification) gave Roshi some trouble and it was stated Yamcha also lost because of inexperience and
wasted movement.
I think 1.5x sounds about right. Jackie was efforlessly dodging Yamcha like Goku tooled the Ginyus. WFF was able to take a serious look from Jackie but still not enough to make him work like he did with Kuririn.