I wouldn’t care that much about the perfect bars. After the tournament has started you (the TD) should be able to change the score of each player, so you simply can get reasonable initial scores after you know your participants.
As far as I know, OGS decreases the initial score by one point per rank in the MC bars. In my opinion a point difference of 2-3 points between stronges and weekest player should be more suited for a tournament with only 15-20 players.
Btw.: I’m not sure if the pairing takes the MCscore into account.