column 1/5

Could you tell me how to create a new type of column.

i need 1/5 – 4/5 . i try with 1/4 & 3/4 + sidebar and the 3/4 is pushback under.

if we can’t have 1/5 – 4/5 , is it a way to modify the size of the 1/4 – 3/4

thanks