It works great. Can you explain why mul 12, subs the princ., then
div. By 66. Is this higher math ??

High-school math, tops.

Start with a problem statement:

Total = January amount
+ January amount + increment
+ January amount + 2 * increment
+ ....
+ January amount + 11 * increment

and solve for "increment".

This approach assumes that you want the monthly increases to be linear.
Other approaches are possible if the monthly increases are supposed to
follow a different rule.


So I guess it can not be done with a function.

There are lots of approaches. I'm sure some would use a function. If
there's some reason why a function is required, please explain.