What I find interesting the most is the Maximum target designation range, which for the S-400 is 390Km for a 4 Square Meter RCS target.
This means that the S-400 should only be able to detect (or at least designate) a F-35 Stealth Fighter aircraft which has a RCS of around 0.001 square meter (on average) at a maximum range of around (and only) 49km or for those more used to Western military aircraft, at around 26.5 Nautical Miles (and this without the F-35 using its jamming/EW techniques).
That's extremely low/short ranged and even more so considering that the S-400 is one of the best and most powerful air defence systems in the world so no wonder why Israel is so keen to procure the F-35 to counter potential and future S-400 deployment on nations around (and hostile to) Israel.

I would not see this picture so bright.
Let me use a target with RCS of 0.006 square meters (this is what I knew for the F-35).

Lets cut the threat the S-400 poses into two scenario:

Scenario-1 (as of today, while the F-35 is under testing)
Most of the fielded batteries in Russia, including the unit in Syria have the following equipment:
S-400 battery with (usually eight) 5P85S3 (like in Syria) or eight 5P85T3 missile launchers.
These units are armed with the 250km ranged 48N6DM missiles.
After the 96L6 all-altitude target detection radar acquires the stealth (with the RCS of 0.006 square meter), the 92N6 multi-functional radar can start engaging it only from 49km. (as you caltulated correctly)

Scenario-2 (proposed for India, future option when the F-35 will be a threat)
S-400 battery having 51P6 missile launchers.
Unit is armed with the 380km ranged 40N6 and the 120km ranged 9M96D missiles.
After the 91N6 target detection radar (in mode M2/M3/M4) acquires the stealth from 94km, the 92N6 multi-functional radar can start engaging with the 9M96D missiles as it has inertial guidance with mid-course datalink and active radar seeker for the last few seconds of the engagement.