Replicate Default Behaviour When Passing Ranges To Functions

I've got this relatively simple file I use to calculate some values for a game mod. Its currently using ad-hoc excel functions, but its maths related to aerodynamics and they get unreadable quickly. I'd like to move to VBA functions for ease of maintenance, and programming the maths part of the code has been no issue so far.

However, I don't understand how to replicate the functionality of the default maths functions regarding range inputs.

Simple example, one of the intermediate values is the Aspect Ratio of the wing:

Code:
=ROUND(2 * Semi_Span / Mean_Aerodynamic_Chord, 3)

This works fine with both numeric inputs, cell references, or, as shown here, defined range names.

However, my VBA version:

Code:
Function AspectRatio(b_2 As Variant, mac As Variant, Optional round As Integer = 3) As Variant
AspectRatio = Application.round(2 * b_2 / mac, round)
End Function
Only works with single-cell references or numeric inputs.

When called as:

Code:
=AspectRatio(Semi_Span, Mean_Aerodynamic_Chord)
It fails.

From my limited understanding - I've only started looking into VBA about 2 hours ago - I need to do a IsNumeric test on the inputs in question, and then use the row from Application.Caller.Address to select the right cell if the input is a range? Am I on the right track?

How to craft a Sub I can call in the 15+ math functions I made.