MnP hosts competing ferromagnetic and helical orders that are highly sensitive to lattice geometry. In Mn${}_{1-x}$Ru${}_x$P ($0\le x\le 0.10$) single crystals, Ru substitution produces an anisotropic expansion of the average orthorhombic lattice, with approximate increases of 0.04~ in $a$ and $c$ and 0.01~ in $b$ at $x=0.10$. Concurrently, the helical transition temperature $T_{\rm S}$ and the Curie temperature $T_{\rm C}$ converge at 215~K from 51 and 291~K, respectively, and the characteristic reorientation field $H_{\rm S}$ at 5~K exhibits correlated increases along [010] and [101], rising from 2.5 to 30.0~kOe for $H\parallel [010]$. A combined analysis with published data for Mo- and W-substituted MnP reveals approximate linear correlations between both transition temperatures and the room-temperature $b$-axis parameter over the investigated composition ranges. Lattice-only first-principles calculations for pristine MnP show that expansion weakens selected ferromagnetic interactions relative to the competing antiferromagnetic interaction, favoring the helical configuration. These results identify anisotropic lattice expansion as an important structural contribution to the stabilization of helimagnetism of MnP.