You can establish fixed-width columns and percentage-width columns in the flex grid using width utilities — .w# and .w#ct.

20x/160px width
.w20
50x/400px width
.w50
30x/240px width
.w30
20% width
.w20ct
33.333% width
.w33ct
fill width
.col-fill
33.333% width
.w33ct
33.333% width
.w33ct
33.333% width
.w33ct
col f1
.col-f1
col f1
.col-f1
col f1
.col-f1