for nav trips to be counted towards the issue of a PPL or CPL, they have to be a "Triangular" flight of at least 150nm with 2 interim stops and all done in a day.
Singapore licensing is largely based on the old UK national system. The flight that you describe is an ICAO requirement for the issue of a PPL (often called the qualifying X-Ctry)
2.3.1.3.2 The applicant shall have completed in aeroplanes
not less than 10 hours of solo flight time under the supervision
of an authorized flight instructor, including 5 hours of solo
cross-country flight time with at least one cross-country flight
totalling not less than 270 km (150 NM) in the course of which
full-stop landings at two different aerodromes shall be made.
As you already have a PPL this requirement has been met and is no longer of any relevance to you.
Navigation flight time is time that can be identified as being on an organised or planned route, from A to B or via a series of way points A to A, via B and C etc rather than just flying around in circles and landing back at A. Think of the person in the Licensing Department trying to verify that you have enough hours; if you fly A to B it is obvious. If you fly a A to A write down Nav and list the turning points.
For a CPL you require: 20 hours of cross-country flight time as pilot-in command including a cross-country flight totaling not less than 540 km (300 NM) in the course of which full-stop landings at two different aerodromes shall be made; it does not have to be on a triangular route neither does it have to be solo, unless you are an Integrated course student. You do not have to return to the point of departure but, because the requirement is for a "flight" not a series of flights, it is considered by most States, that completing this requirement in the course of one day (24 hours) meets the spirit off the requirement.