Three columns, total width is 180.
First column has the width of fieldC 2/3 = 90
Second has the width of an empty label 2/3 - 1/2 = 30
Third has the width of labelA 1/3 = 60
[ ][labelA] 2/3-1/2, 1/3
[ fieldA ][fieldB] 2/3, 1/3
[ fieldC ][ fieldD ] 1/2, 1/2
The point is to make the second column a size of 2/3 - 1/2 for the design to appear as we want. I have used a component that prints nothing for that. If we let that component with weightx 0.0 , the relationship among the columns of the grid will vary when resizing. Setting it to 2/3 - 1/2 of 1.0 keeps the proportions, also, in that case.
a) by adding more rows with "dangling" components we can overcomplicate the design with columns holdings invisible components.
b) by adding more rows, a component may be placed in a column for which its weightx setting is incovenient. Say, a component in the first column below fieldC, whose weightx is 0.7, will make the weightx of the first column 0.7 ; which is incompatible with our necessity of 0.5 for that column.
The point is to make the second column 20% of the space being used, whatever that may be. Since the Label takes up the entire space being used and has a weightx of 1.0, 0.2 should be 20% of that space. 0.5 should be 50% and 0.3 should be 30%. The right column has to be 0.3,