How to change Zen Grid System
$zen-column-count: 10;

.container {
 @include zen-grid-container;
} 
.column1 {
 @include zen-grid-item(3, 1);
}
.column2 {
 @include zen-grid-item(7, 4);
}


You include 10 columns, and use 3 for 30% and 7 for 70%. Reason being that columns are of equal width and if you want to implement two unequal columns you have to use multiples of a common denominator.