I'm guilty of wasting my SIN SKL F access ~90% of the time. The 10% of the time I'm using it is when I travel for leisure and of course, I won't be just by myself.

Nevertheless, I can say that 20 mins should be enough. Because I usually start stepping in to clear immigration only ~20-25mins before the flight. That is if I see no or very short queue in the immigration.

Net, if I can always make it by clearing immigration only 20mins before the flight, you can make it from SIN SKL at quite the same time. Of course, I walk fast and I do not stop for duty-free shopping. With this timing I'm not even the second or third last person to step into the plane. Usually there are at least 5-6 people after me.