In September of 2013, JAXA launched an $80,\!000\,\mathrm{m^3}$ zero-pressure Helium balloon from Hokkaido[<sup>1</sup>][1]. It reached a float altitude of $53.7\,\mathrm{km}$.

From the ICAO 1993 Standard Atmosphere[<sup>2</sup>][2], the density at that altitude is $6.6\times 10^{-4}\,\mathrm{kg/m^3}$.

That same zero-pressure balloon would float at the same density at Mars. The altitude equivalent to that density will vary over the Martian year, since the mass of the Martian atmosphere varies $\approx\!25\%$ over that time! Since I have MER atmosphere models handy, I find that that density was at an altitude of $\bf 35.7\,\mathrm{\bf km}$, at an L<sub>S</sub> of $328^{\Large\circ}\!$, about two-thirds of the way into Southern Summer (the time of the Spirit landing).

Note that what matters here is density, not pressure. A zero-pressure balloon is an accurate densitometer at float altitude. Also, the difference in the compositions of the atmospheres, in particular the average molar mass, is already accounted for in the density.

  [1]: https://www.researchgate.net/profile/Hideyuki_Fuke/publication/315920020_Recent_Highlights_of_Scientific_Ballooning_in_Japan/links/5bb62c8392851c7fde2e763a/Recent-Highlights-of-Scientific-Ballooning-in-Japan.pdf
  [2]: http://www.aviationchief.com/uploads/9/2/0/9/92098238/icao_doc_7488_-_manual_of_icao_standard_atmosphere_-_3rd_edition_-_1994.pdf