[Mart]

I might be wrong but overlap is something associated with performance engines. When I observe a flathead with a stock cam the exhaust closes then the inlet starts to open. They are not both open at a similar height at the same time.

But the bottom line is that the point at which the exhaust has just closed and the inlet is just about to start to open is the point at which the piston should be at TDC.

For reference, No.6 valves behaving as described above will coincide with No.1 at TDC having just fired. (On the firing stroke).