A post the other day on one of my photo boards got me thinking on finding out what is Lightroom's definition of what is blown and how does that compare to the threshold at which printers run into issues.
Various print companies (WHCC, Miller's etc) have admitted to having issues printing whites that fall between 230-255, so you need to give yourself some room away from 255 if you want to retain detail in an area when printed. What specific value you limit to will be a judgment call you will have to make based on where you print (and where your clients if you have them may print). There is a chance that something will look fine on your screen but like crap when printed, and you want to produce images that look great in both situations right?
I made a little test document in PS to compare where the threshold mask would tell you that you were at risk of losing detail to compare to the highlight alert in LR. Here are my results
**Click for larger**
Looking at this you can see that Lightroom has the limit where it alerts you in red for blown highlights (hit the J key to toggle this on and off) VERY high compared to a threshold mask in PS set to 240. To me this says that we can't just back off a little bit from having the red alert in Lightroom, we need to be well away from that.
Lightroom also isn't going to tell you if you blow just one channel that should be there 1) it doesn't know it should be there and 2) it seems to only look for pure white. Lightroom's highlight alert is going to be no help in telling you if you blew the red channel in your skin for example. I hope this information is useful!