I think, if you know your usual conversion efficiency, you could use this modified version of Sean's formula to get a more accurate estimate of your mash efficiency from a no-spargeI would argue that if you know your conversion efficiency is low, you should work on fixing it, rather than work around it.
I would argue the same thing, but, from all the 75% efficiencies I see listed from sparged beers, I wouldn't count on it.
I was speaking about overall brewhouse efficiency - I have no idea what my conversion, etc. are.
It's quite useful information to have when you want to do something like, as in this case, predict the actual gravity you will see from a No-Sparge mash. Like Sean demonstrates, the math gets even easier once you get close to 100% conversion efficiency and can ignore that factor.
The difference between the expected 75% mash efficiency and a 55% Brewhouse Efficiency seems like it would be due to an awful lot of deadspace, but I know that many brewers systems leave a lot of wort in the kettle. I don't usually calculate BHE, because it is not a number that I have ever found a use for other than figuring the cost of a batch. However, sitting down and doing the calculation, it would be ~70% for a No-Sparge batch, similar to the 5% loss that Morticaixavier sees. He is right, too, about the fact that a No-Sparge mash will give you significantly higher efficiency than a Parti-gyle mash, which may account for the differences.