500mA should be fine, that's the minimum I recommend for it. If it flashes 5-6 times then stabilizes then it shouldn't be a huge deal, I would only started worrying if it's taking more than about 20 seconds to stabilize.
Power supplies are complicated because they're all designed differently. 400mA is closer to what the pedal will pull when it's actually warmed up, but some power supply manufacturers will limit their supply exactly at the rating while others will allow a little "wiggle room" in the output. I know that OneSpot, for example, can exceed its output rating for a brief period of time without damaging the supply. I've pulled about 1900mA out of a 1700mA OneSpot supply for a few seconds while tubes were warming up. If a supply is hard-limited at 500mA then it should still work, it might just take a few seconds to get started, as you observed.
This type of thing is exactly why I started offering power supplies as an optional add-on for my pedals. The supplies I carry are nothing special, you're not required to use them, they don't make the pedals sound better or anything like that. But they're filtered well so the power is quiet, and they're rated for 1000mA, so they're a safe option if you don't already have a supply or if you don't know if your supply will work.
I currently only have US-style plug power supplies, but I am working on getting some international supplies with swappable plugs so I can provide supplies to my friends elsewhere in the world.