Low mass cut-off for the Dharma Planet Survey's detection of habitable planets around 40 Eridani A?
In general, finding the low mass cutoff of a particular RV survey is normally quite complicated as there are a lot of selection effects on many different variables (such as planet period, planet eccentricity, host star type, signal-to-noise, stellar activity level etc) that determine whether you can recover a planet signal. In the case of this paper, this would be particularly true as they are combining datasets from several instruments, each with different coverage, sensitivity and systematic noise properties. The way this is normally handled is through a "bootstrap" technique where you inject planet signals of known parameters and see what your detection code finds and study the recovery percentage as a function of the varied parameters.

Using the online Habitable Zone calculator for a Teff=5072 K, Stellar luminosity=0.47 * L_Sun (luminosity of the Sun) matching the host star, produces 0.678 AU as the inner edge of a conservative habitable zone. If we use this, the star's mass from the paper (0.78 M_Sun) and the 1 M_Earth mass you would like to find, then making use of the formulas given on the Exoplanet Archive website, gives us a period of 230.9 days. Plugging this into an equation for the radial velocity semi-amplitude (also on the Exoplanet Archive), produces an amplitude of about 12 cm/sec. This is assuming an edge-on orbit, so the sin i term from the inclination is 1; given the lack of detected transits in the paper, the inclination is likely lower than 90 degrees, which would lower the RV amplitude further. 

This RV amplitude is far too small to be detected with current instruments. The best instrument we have right now is HARPS on the ESO 3.6m telescope, whose data is included in the paper. If I am understanding their modeling correctly, they get error bars of 181 cm/s on the HARPS data for this star in their analysis, 15x the expected signal. Also note that the calculated RV signal is somewhat best case (unless it is more massive); decreasing the inclination from edge-on (likely) and moving the planet further out in the habitable zone from the inner edge I calculated (also likely) will both decrease the RV amplitude further. Even for future instruments such as ESO's ESPRESSO which will go the 8.4m VLT and reach 10 cm/s precision, this would be a challenging measurement.