Can I use long(er) variable names?

[pinkman]

Once you created you variables FOCAL and DIAMETER:

RATIO:=FOCAL/DIAMETER will define a variable and assign a value for it, without being updated each time you change FOCAL or DIAMETER.
You can create a function (shift Define) named RATIO and give the expression FOCAL/DIAMETER in the function field. Then uncheck the two checkboxes and now in home you can use RATIO, it will be updated each time you modify FOCAL or DIAMETER.

If you need to solve one of the three values knowing two of them, delete your previous RATIO function, create a variable named RATIO, then use the Solver App with the expression RATIO=FOCAL/DIAMETER