To get the max 20K bonus, you would have convert enough hotel points to get 80K miles.
100K UA miles for:
800K Hilton
160K SPG
400K IHG
800K Club Carlson
400K Choice
400K Wyndham
80K Gold Circle
Hyatt: 200K Hyatt -> 120K UA (80K miles + 20K bonus on transfer + 25% promo)
Marriott (max 50K miles per year): 112K MR -> 62.5K UA