  We model the normal state of ferromagnetic superconductors with two general
ellipsoidal Fermi surfaces (FSs), one for each spin projection
$\sigma=\{\uparrow,\downarrow\}$, each with its ferromagnetically split
chemical potential $\mu_{\sigma}$ and its three distinct single particle
effective masses, $\{m_{i\sigma}\}$, the geometric mean of which is
$m_{\sigma}$. We study this model in the presence of an arbitrarily oriented
magnetic induction, ${\bf B}=\mu_{0}{\bf H}+{\bf M_{0}}$, where ${\bf M_{0}}$
includes the Ising-like spontaneous ferromagnetic order, which for URhGe is in
the $c$-axis direction above the superconducting transition temperature $T_c$.
We assume the low-$T$ total particle density $\Sigma_{\sigma} n_{\sigma}({\bf
B})$ to be independent of ${\bf B}$, and obtain a self-consistent asymptotic
expansion for $\sum_{\sigma}\Pi^{3/2}_{\sigma}({\bf B})$ in even powers of
${\bf B}$, where $\Pi_{\sigma}({\bf B})=m_{\sigma}({\bf B})\mu_{\sigma}({\bf
B})$. We assume that the $\mu_{\sigma}({\bf B})$ are linear in ${\bf B}$ for
both spins due to the Zeeman interaction and that the remaining even ${\bf B}$
dependence in the $\Pi_{\sigma}({\bf B})$ arises only from $m_{\downarrow}({\bf
B})$. Our analogous expression for the Sommerfeld constant $\gamma({\bf B})$
leads to good fits to the $\gamma({\bf H})$ data of Aoki and Flouquet [J. Phys.
Soc. Jpn. \textbf{81}, 011003 (2012)] obtained for the ferromagnetic
superconductor URhGe in the ferromagnetic, non-superconducting phase, with the
applied magnetic field ${\bf H}$ along each of the three crystallographic
directions. We discuss this model in terms of the reentrant superconducting
properties of URhGe and UCoGe. This model can be generalized to an arbitrary
number of ellipsoidal FSs.
