Holder did a pretty good letter. Screw a three page letter, all you really need is acknowledgement by Holder and therefore the President that SCOTUS has the authority.
Perhaps I'm an odd bird, but I saw Obama's original comments as not so much an attempt to pressure SCOTUS - at which they would abysmally fail - as an attempt to pre-define an expected decision as judicial activism, an attempt to redefine a SCOTUS defeat in favorable terms as a campaign issue. Accordingly, I didn't see the remarks as anything out of the ordinary, just politics as usual - arguably a stupid thing to say, but not an attack on the judiciary's authority. For that matter, I still don't predict that SCOTUS overthrows the mandate, only the asinine idea that it can be a tax when needed and not a tax when needed.
Interestingly, Holder and Obama want SCOTUS to throw out DOMA, which actually did pass by a large and bipartisan margin rather than by a baseline party vote. (This is of course also proof that large and bipartisan margins are not proof against stupidity and perhaps soon, that they are not proof against unconstitutionality.) Were Obama to intend his words as pressure on SCOTUS he'd also be undermining his efforts on DOMA.