Simple question I cannot get right:

A=10

B=50

A+B = 60 (60 is a 100%)

What is A in %

A / 60 = 0.166...

0.166 * 100 = 16.66%

How would I put that in one calculation without using an extra field?

IE. I can get it like this:

field_A + field_B = field_C (A + B = C)

(field_A / field_C) * 100 to get %

The question is how to set up the calculation without the use of field_C?

How about:

A / ( A + B )

You can format the field to display as percent.