Something like this:

    auto
    operator"" _rpmPerVolt (long double x) {
        return decltype (1_rpm / 1_V)(x);
    }

I have a macro for that:

    #define SI_DEFINE_LITERAL(xUnit, xliteral) \
     [[nodiscard]] \
     constexpr Quantity<units::xUnit> \
     operator"" xliteral (long double value) \
     {  \
         return Quantity<units::xUnit> (value); \
     } \
     \
     [[nodiscard]] \
     constexpr Quantity<units::xUnit> \
     operator"" xliteral (unsigned long long value) \
     { \
         return Quantity<units::xUnit> (value); \
     }

    // Base SI units:
    SI_DEFINE_LITERAL (Meter, _m)
    SI_DEFINE_LITERAL (Kilogram, _kg)
    SI_DEFINE_LITERAL (Second, _s)
    SI_DEFINE_LITERAL (Ampere, _A)
    SI_DEFINE_LITERAL (Kelvin, _K)
    SI_DEFINE_LITERAL (Mole, _mol)
    SI_DEFINE_LITERAL (Candela, _cd)
    SI_DEFINE_LITERAL (Radian, _rad)

...and a loong list of other common units here.

The additional types are defined like this, in another file:

    using Foot          = ScaledUnit<Meter, std::ratio<1'200, 3'937>>;
    using Mile          = ScaledUnit<Meter, std::ratio<1'609'344, 1'000>>;
    using NauticalMile  = ScaledUnit<Meter, std::ratio<1'852, 1>>;
    using Inch          = ScaledUnit<Meter, std::ratio<254, 10'000>>;

And then I use SI_DEFINE_LITERAL (NauticalMile, _nm); etc.