AWK "make a new column that include increasing numbers"

please help!!!!!!
I have a file .txt that has only one column like that:

34.1
35.5
35.6
45.6

...
Now, i want to add a column in the left in which the values of this column increase by 0.4 , for example:

0.0  34.1
0.4  35.5
0.8  35.6
1.2  45.6

How can i do with awk instructions???
Many thanks!!!!!!!!

awk '{printf("%.1f %s\n", 0.4*(NR-1), $0)}' infile

Thank you Kevintse!!!!!!!!!!!!!!! i got it!