It seemed perfectly reasonable the way it was supposed to work -- a multiplicative bonus (before crew exp bonus) that increased per 5s increment (up to five or six times) as long as you kept the missile continuously on sensors.

So, for example, if you had a 30% chance to hit (+5% for crew XP) and a 10% tracking time bonus, after one 5s increment you would have a 33% (+5%) chance; five seconds later 36% (+5%); after 15s total tracking time it's 39% (+5); etc.; to a max of 45% (or 48%, I forget) +5% for crew grade bonus after 25 (or 30) seconds.