The theory of sequencing is fairly straightforward. An untouched landing time is calculated for each aircraft (system estimate for the fix plus a calculated time from fix to threshold based on STAR tracking, aircraft type and winds). This generates the order of arrivals.
No. 1 is assigned his untouched landing time, then everyone follows along with however many seconds spacing between arrivals as specified by the acceptance rate.
Your sequenced landing time is then used to back calculate the fix time needed to achieve it. Manual flowing (no Maestro) works pretty much the same way.
The 250NM is simply the nominal range of radar - the flow only sees the TMA radar feeds & you can't flow what you can't see. The further out you implement a solution the more subject it is to interference.