You take your raw time for each stage and add to it the points down for less that -0 hits.
(A shot in the center circle or the head is no points down [-
; a shot thats in the area surrounding the center area [circle] is down one; the next area outwards is down 3. Misses are down 5.) The sample target in the rule book shows these areas. You can also be penalized for not getting at least 4 points on the target (i.e., a failure to neutralize.) If you only hit one shot in the -3 area, you'd have 2 points -- and a "failure to neutralize penalty" on top of the other points down for missing.
Add all your points down together, multiply by .5 and add it to your time, and that's your stage total.
In a match, you can also be penalized for doing things wroing (procedural errors), for hitting a hostage, or for bad sportsmanship. Penalties don't get multiplied by .5.