SQL add column based on if statement

I have a table with a 4 character (col1) that I would like to:

• split in half
• use an if statement to recode the left 2 characters as specific integers
• concat left onto rght (separated by '.') to generate the fin column below.

col1	left	rght	fin
xx01	xx	01	01.1
xy01	xy	01	01.2
xz01	xz	01	01.3

Bit of a SQL noob so appreciate any advice. Thanks!