Origin / reference point with different quantity type

[oZed]

I am trying to use mp-units for some hydraulic calculations and so wanted to implement the hydraulic head as a new quantity. It is somewhat common to calculate the head in terms of either the fluid in question (sometimes called the 'fluid' head) or normalised against the specific gravity to give the head in terms of water (sometimes called the 'water' head). I want to make these two quantity types distinct to prevent them being mixed up, however since the fluid head really acts like a tuple of quantities (a head and a specific gravity value) it isn't really modelled by the ISQ character types. I thought about trying to use a quantity_point with the specific gravity being somewhat like an origin, but currently the library requires the origin and delta to have the same quantity type. Am I better off just implementing a custom type in such a case? Please let me know if I've missed something and this can be implemented with the current library types!