We've recently raised $500k pre revenue on a startup.

There is not a 'right time' because every investment avenue, fund or 'investor' is very different. You need to understand what type of investors you're pursuing first and then determine what their requirements are. Only then will you know what the right time is.

If it helps, I like to describe it as a black box of confusion, with very little transparency and tons of time wasted going after the wrong people. The more you can understand what's out there, you can better tailor your pitch and timing around your options.