How does blockchain.info calculate the "Estimated Transaction Value?"
For transactions with one input it is hard to guess what is the change and what is the real payment. However because they may be have statistics so they can may be guess that for a ~4 BTC input there are 70% of probability that the change is the bigger output and the money spent the smaller one when both output addresses are new.

Now if you have a transaction with several inputs (Like ou have already explained in your question) it is much more easier to guess.
If you have :

As input
1Pi87D6NUPB5BxcuowYN4Lj3WLCSFGsuLN (0.24379822 BTC)
1PgBMDggd7GGCifDd1881vdvNe8pkn5BWk (0.08787591 BTC)

And as output
1B6tvnE3oP3mwoA4RMrYXWVup4Z73G6bDw -  0.05377413 BTC
1DswmdznL6exZzN6akJpsPUc4ZWRZo9JQc - 0.2778 BTC


In this case you can be pretty sure that the spent address is '1DswmdznL6exZzN6akJpsPUc4ZWRZo9JQc - 0.2778 BTC' and the change '1B6tvnE3oP3mwoA4RMrYXWVup4Z73G6bDw -  0.05377413 BTC'.

Because otherwise there would be only one input.