Usain Bolt, a superstar athlete, but one with (more or less) human powers rather than a superpower, demonstrated almost one "g" of acceleration at the start of the 100m in the Olympics. A superhero capable of sustaining the same force would top out at 120 mph standing tall (belly to air flow), with higher speeds (but still well below 180 mph) as he leaned forward and ducked his head.
Since the Flash should have a suit that provides laminar air flow (no turbulence), air resistance for the Flash would be proportionate to velocity. If his superpower only allows him to maintain maintain the peak exertion that a top athlete can achieve for a fraction of a second, we have an estimate for his speed. If he also has a form of super-strength (not comparable to Superman or the Hulk, but still stronger than even an Olympic Athlete), he would need quadruple the sustained leg power of Usain Bolt's first half second to hit 240 to 300 mph, and sixteen times the sustained power to hit 480 to 600mph. I would not trust calculations beyond 600 mph, but we can estimate the Flash's leg strength from those numbers and comparing his speed to that of known objects he can outrun.
That's for normal physics, which was your question. Since the Flash can run in space, run on air molecules, and run at "multi-light speed", I imagine different equations, far beyond what we have discovered outside Metropolis and Central City, take over when he wants to go really fast.
