That's why I love Singapore - at SIN, it's 10min before departure, no matter what.
Sure, Gate Security might be a bit annoying, but it gives you MUCH more flexibility.
Exit the MRT (subway) 20min before departure? No problem!
I've (not by intention) made it by arriving 15 minutes before departure.. of course it was a mad rush, but I've made my plane. And I knew I'll make it (at least after I got through passport check, but never had to wait longer than 30 seconds when exiting Singapore anyway) - you reach the gate security 10 minutes 5 seconds before departure, and they'll take you.
I can't believe why other airports still fail to have such a fixed "gate closure" system as Singapore. It simply works perfectly, and it wouldn't take anything to implement it.