Last time I looked a square wave had a 50% duty cycle or equal mark space ratio as you so quaintly put it.
I agree that a typical square wave has a 50% duty cycle. I think the case pebe is refering to is important for digital signals where the duty cycle may be anything between 0-100% for a given period. For example: If the total time period of the digital signal is x The digital signal is at Vmax for y periods of x And the digital signal is at Vmin for z periods of x Where x = y + z Then the DC offset is calulated by: (x*Vmax + y*Vmin)/x This is a general solution of which the one Papabravo gave above is a spcial case where the duty cycle is 50% Dave
When I asked joka2188 for a few more specifics, I was looking for a confirmation that the square-wave in which he was interested had a 50% duty-cycle. While all answers thus far provided are accurate, the general case that Dave has provided covers all contingencies nicely. hgmjr