It has been a good calendar year for the rebuilders.

The 2015 rookie class has been one of the more exciting ones for fantasy in recent memory, and their stories should only grow more compelling in time. Consider this. Six members of Keith Law's top 100 prospects from the preseason have debuted in not only the majors, but also shown us, whether for shorter or longer stretches, glimpses of what they will be in the prime of their careers. Even more interesting, those six don't include Byron Buxton, Addison Russell, Francisco Lindor and Blake Swihart, all of whom have debuted this season and are practically untouchable commodities in dynasty leagues.

There is no shame in rebuilding, and it is classes like this that can make such a strategy a quick one. Remember, as I have said before in this space: Knowing when you can't win is as critical a skill as winning in a keeper league in the first place.

Perhaps what is just as important as adding those final pieces to your championship puzzle is recognizing two stages: The first, when your keeper-league roster is doomed to fall short in its championship quest in the given season; and the second, when your keeper league's trade market is soft and sure to offer the richest prospect haul. These are not necessarily coincidental. That is why the ability to recognize both is a critical skill for a keeper-league owner.

Generally speaking, this is the time of year you can expect either of those stages to arrive. Trade season is heating up, especially with the ESPN trade deadline less than a month away.

So to give you a head start, let's refresh my Keeper Top 250 rankings, which serve as an effective "price guide" for those evaluating trades in keeper leagues.

The rankings formula

You know the drill: It is impossible to craft a one-size-fits-all set of keeper rankings, because few keeper leagues are identical in structure. Beyond the obvious roster size and scoring system, these are some of the variables:

[b] Player pricing: Do you draft or auction players, and do you keep players in the round they are picked, for the auction price paid, or are prices irrelevant?

[b] Number of keepers: Can you keep one, five, 10 or perhaps your entire roster, and must all teams retain the same number of players?

[b] Contract factors: Is there a limitation on the number of seasons you can keep a player, and is there annual price inflation?

[b] Farm teams: Does your league include minor leaguers, and do these players automatically carry over, or is there a cost for keeping them?

[b] Team competitiveness: Are you a contending team, a rebuilding team or are you somewhere in between? At midseason, the answer to this question is far more critical than it is during the offseason.

Your homework is to first assess each of these valuation factors, recalculating, if necessary, these keeper rankings for your needs. For example, there's no way I would trade a $6 Christian Yelich for a $37 Jose Bautista if I was not in contender mode, despite the fact that Bautista ranks 11 spots higher. As a contender, though, I would seriously consider it.

This is the player valuation formula that I used:

• 2015 second-half performance: 10 percent.

• 2016 performance: 22.5 percent.

• 2017 performance: 22.5 percent.

• 2018 performance: 22.5 percent.

• 2019 performance and beyond: 22.5 percent.

The rationale for these numbers is simple: Equal weighting for each of the upcoming four seasons as well as the remainder of this one. As for why 2015 -- scaling that percentage to remaining games -- is roughly equal in weight to 2019, bear in mind that I already provide updated rankings for fantasy owners focusing solely on this year. They are published weekly and the last edition can be found here. If you're a keeper-league owner still in the hunt in 2015 -- that's always my recommended strategy, if you have even the slightest chance of winning -- consider this list more of a "price guide" for determining appropriate trade value for your up-and-coming prospects and youngsters.

Tristan's Keeper Top 250

Note: Position eligibility ("2015 Elig. Pos.") is determined based on a minimum of 20 games, the position the player appeared at most often in 2014, or 10 games played at a position in 2015. Players' projected future positions are considered in the ranking. Players' listed ages are as of July 14, 2015.

Players' 2015 preseason ("Pre 2015") and peak rankings in past keeper lists ("Prv. Peak") are also provided: These lists have been published semiannually since 2010, with "preseason" and "midseason" designated to differentiate the different times of the years in question. For example, Adrian Gonzalez is listed with a peak of 10 at "Pre-11," meaning that his best all-time rank was 10th, in the 2011 preseason list. A "--" means the player has never made a Keeper Top 250 rankings list.