My procedure is to let the ferment tell you when it's time for the diacetyl rest. At some point (usually at or below 50% ADF) you'll notice that the activity (bubbling etc) starts to slow, and, if you can monitor it independently (not sure what clues the brew jacket gives you,) you'll see that the ferment is putting out less heat, and you need to reduce your cooling load to maintain the same temperature in the ferment. This tells you that the yeast has gone through primary (eating the easy simple sugars) and is transitioning to secondary fermentation (consuming more complex sugars and previous fermentation byproducts.) At this point flavor is set, and you can let the temperature free rise to move things along. This is more sensitive to the actual life of the yeast than using an arbitrary % attenuation, since you may not be able to accurately predict your final gravity (especially in a big beer.) And remember, WY 2124 can run warm and still be clean! Fear not.
BrewJacket's pretty cool. I've been brewing since '94 and just never bothered with lagers, because I didn't have the space for a refrigeration unit and couldn't lager at temp. (new studies aside, regarding lagering at room temp.) When I bought the BrewJacket I primarily went about the business of continuing to make ales, but I'm finally getting around to lagers.
Annnnway, it does let's you know what the temp is at, as the yeast turn the malt into alcohol and the temp steadily drops. I use a refractometer to take my gravity readings, don't really depend on the bubble method any more.
I was going to check at week 5 for any D by doing the microwave heat test and if the beer was in great shape, I was going to push it through a 5 micron filter, keg and force carbonate it and then bottle it.
I'm clearly jumping the gun because I've got an entire other yeast starter for a Czec pils I'm dying to make, but just need to relax and have homebrew.
Thanks everyone for your help.
Cheers,
D