They're generally excellent, but garbage in, garbage out: they rely on accurate data being input. I am skeptical that you will get 4K PQP from a round-trip on SQ.

Note that if you are flying one of SQ's flights with an en route stop, such as JFK-FRA-SIN, if it's a through flight (same flight number), then the stop will be omitted, you'll earn on the nonstop distance (JFK-SIN, in this case), and you'll be capped at 1K PQP for that leg.