Hi there excellers, can someone help me with this problem

Col a Col b Col c Col d
predicted time handicap total time
row1 0:18:00 100% 0:09:00 0:27:00
row2 0:27:00 0:00:00 0:27:00
row3 0:22:30 0:04:30 0:27:00
row4 0:20:00 0:07:00 0:27:00
formula h:mm:ss h:mm:ss h:mm:ss
I am trying to do handicaps and what I need is a formula.
Row 1 col a is the fastest time which I want as 100% so that the other
handicaps will be based on the fastest time, how do I get row 2,3,4
col a to be a percentage of Row 1 col a
Regards Ditchy
T.I.A