Thanks to Jon Custer, now you have a full set of densities against mole fractions of chloroform and n-heptane mixtures in this site. Just for my curiosity, I have plot the relevant graph:

The data is given for temperature at $\pu{25 ^\circ C}$ and the set is in good agreement $(R^2 = 0.9994)$. Since $x$ in the equation is the density, and assuming density of water at $\pu{25 ^\circ C}$ is $\pu{1.0 g mL-1}$, you may get that density by mixing $(-1.1887 + 3.727 - 1.9736) = 0.5647$ mole fraction.
I also found a reliable full data set of densities against mole fractions of chloroform and n-hexane mixtures as well (Ref.1). Just in case if you want to use hexane instead of heptane, I included that as well:

Reference:
- Mehdi Hasan, Ujjan B. Kadam, Apoorva P. Hiray, and Arun B. Sawant, "Densities, Viscosities, and Ultrasonic Velocity Studies of Binary Mixtures of Chloroform with Pentan-1-ol, Hexan-1-ol, and Heptan-1-ol at (303.15 and 313.15) K," J. Chem. Eng. Data 2006, 51(2), 671–675 (DOI: https://doi.org/10.1021/je0504459).