As SekeRob said it is a VERY complicated formula that is STILL evolving with each new release of Boinc. Throw in the fact that instead of just cpu's people are now able to use their gpu's too to crunch with and the potential problems can skyrocket! VERY basically...you choose the amount of time you want to cache units on your pc, meaning you don't have to download a new unit every time you finish the current unit, and then Boinc tries to maintain that amount of work on your pc. Boinc will only get new work when it is almost out of the work it currently has cached on your pc. This does two things, it keeps the Server connections to a minimum, we users are many while they only have so many ports for us to connect to, and 2nd it keeps our pc's busy even when the Server has a hiccup and runs out of work or is down for a limited amount of time. I would guess that MOST people with cable type internet connections try to keep a smaller cache, while those with other types of connections tend to have a larger cache. My personal cache is about 1 day but I do extend it when a project has a planned outage coming up, just so my pc's can keep crunching. Too long of a cache though can cause its own problems because each unit has a deadline and if you miss the deadline you probably will NOT get credit for the unit. In that case it is better to abort the unit and just get a fresh one from the Server, the Server will just resend your uncrunched unit out to another pc.

Part of the 'high priority' crunching is that you probably run other Boinc projects too, causing an abundance of work that Boinc thinks it can't finish before the individual workunit deadlines. If this is true you should consider aborting some of the units, if it isn't then Boinc WILL figure it out eventually, although it can take a while. Although Boinc has been around a LONG time it is still not very good at some things, which is why there are always new versions coming out. Some of those work well, some don't, which is why WCG 'certified' a version that works for it. In actuality MOST versions will work with WCG, some just MUCH better than others. IF you ONLY crunch for WCG stick with the 'certified' version, if you do venture out into other projects it is what it is.

Don't abort.
My machine runs at high priority all the time and won't miss a deadline unless it's turned of.
Unless a task's deadline is a few hours away and at very low percentage complete, don't abort any tasks.
Also, if it's on high priority, it doesn't mean that the tasks that are being processed won't finish in time but rather the ones that haven't started yet. So lower your cache.