OK... that's the way it should be done.
I s'pose people just aren't catching on that we're using the TDC to compute a lead angle for a 0°-gyro shot. The only thing we've done is eliminate a lookup table for different torpedo/target speeds.
We should do an advanced O'Kane method that allows folks to approach from different angles other than 90°. Then the torpedo advance would actually produce an ever-so-small error associated with the distance estimate. I'd still just set it to 1000-2000 and not worry about it though. You might get a kick out of watching your torpedo turn into the target from a 45° approach angle.