When determining handicap for a mixed league, there are two main issues to contend with:
A) A handicap system that is too low makes it difficult for lower average bowlers to compete for a league title;
B) A handicap system that is too high rewards sandbagging, especially for sweepstakes.
The end result of both of those situations is people choosing not to return for future seasons because they believe they are not getting a fair shake. So to determine the best way to resolve those issues, we need to look at what the current trends are, if they fall into either of the above categories and how they should be modified.
Many leagues now have handicap systems in place that are 90-100% of 220-230. This is too high of a handicap system. Bowling has a finite score system (0-300), so a 215 bowler has much less room for improvement (shooting above 215) than he does for decline (shooting less than 215). Conversely, the 120 average bowler has the opposite more room for shooting above average than below. For equity, this demands that a handicap system be less than 100%. Further, it takes an understanding of how scoring works in bowling to realize that lane conditions come into play in order to be able to average over 200. On a tough condition, a good bowler who stays clean and gets 30 marks with no doubles is in the 570's, a 190 something average. On a easier condition, that same bowler can easily shoot 700, an average in the 230's. Unless you want the league title to be determined by the lane conditions, you need to err on the lower side of the handicap system, so that skill & performance prevails.
It is a given that on any given night, a lower average team can beat a higher average team even with a lower handicap system. However, over the course of a season, a higher average team will end up winning more often. If we are going to accept that a lower handicap system is better for producing results that reward performance, we must structure the league to give the lower average teams an opportunity to overcome that. The simplest solution is a split league, where the winners of 1 half play the winners of the 2nd half. The league title then comes down to one series and as everyone knows, that makes it very possible for any average team to win. While I have been on teams that had the most total wins for a league but never won a half, it was no different than being on a baseball team that had the best regular season record & got bounced in the playoffs.
Finally, sweepstakes, being a one week "league" with money at stake demands a lower handicap system. The opportunity to have a major advantage by sandbagging is too great for some when a too high handicap system is in place. Since it is only one series, the lower average bowlers have a very good chance to win if they perform well while the higher average bowlers are not shut out if the lane conditions that night are less conducive to high scores. This also comes into play for leagues that travel (to Las Vegas or Atlantic City) for sweepstakes and bowl on lanes different than that which the averages where established at.
Based on the above, I have found the most fair and equitable handicap system is 90% of 200 to 210. When combined with a split season for the league championship, it discourages sandbagging, rewards performance and improvement (even among higher average bowlers) and gives everyone a fair chance for both the league title and sweepstakes.