Changing layout grid independently

I made a 1200px wide artboard. In it, I want to create a 12-column grid, each 70px wide with 30px wide gutters. Logically, it would fit nicely if the right and left margins are 15px wide, but getting it to my specification is a struggle on its own. With all the input fields available (number of columns, column width, gutter width and margins) it should be intuitive enough, but each time I enter a value, it changes the other values.

Logically, the application shouldn't even have to guess (miserably) what the user are trying to do. It should just accept the input values then center it with in the artboard unless the user chose to offset the margins right or left.

{Renamed By MOD}