To whomsoever this may concern

I write to recommend Mr. Dandan Dandanadevan for the Graduate Program in Arachnomatics at your University. I offer a second year course in advanced photography at the Chennai Institute of Technology and also head the photography laboratory. I have been Assistant Professor at this Institute for the last 25 years. I have over 250 research publications to my credit as well as innumerable journal articles. But I digress.
I have definitely seen this guy Dandan somewhere. Was it at the annual cul-fest? No, that guy was thinner. I can't place him exactly but certainly looks like a decent chap.
So, CIT takes in 500 students every year. I remember about 2 students in every batch by name and 20 more by face - the ones who take my course and show interest. There are about 30 more in the grey area who register for the course and meet me only during the end term examination. And then the other 450 students of the batch who I run into in the corridors. I swear Dandan's not in the first 22.
But I urge you to consider this problem in a practical manner. From historical data, 100% of all students requesting me for a recommendation letter have taken my course. The only way Dandan could be otherwise is if he knew I would think along these lines and is exploiting my complacence. Let's say the probability of that happening is 1%, a very aggressive estimate.
I give only B and C grades to my students. About 90% of the reco-letter seekers are "B-putters". C putters come to me if they have flunked just about every other course or if they have done something extraordinary that they think I should remember them by. In either case, out of sympathy, you may consider this letter to be a strong recommendation.
So Dandan has an 89.1% chance of having taken my course and obtained a B grade. This is of course in addition to a 10% chance of being in one of the college sports teams and a 2% chance of being a coordinator of the cul-fest, and so on.
Reducing this based on data of a perfectly normal dude going crazy in the US, I have decided to give you an 80%-correct assurance overall that Dandan will be an asset to your university just as he was likely to have been to ours. If I'm provided more data on how I know him exactly, I could better my estimate.