Simplest way to find the true bearing when the relative bearing is known is by looking at the sonar;
In this example the relative bearing is 090 (red line), since the sub isn't heading exactly north if you want the true bearing to draw on the chart with the protractor, you read that off the inner ring next to the relative bearing on the outer ring, in this example the true bearing is about 067 degrees true. If the relative bearing was 154 (white pointer) the true bearing would be 128.
As for the PK (Position Keeper) click the little white button on the lower right corner and it will turn red. Assuming the target is not zigzagging and doesn't change course or speed, the TDC will continue to update the calculated position of the target while the periscope is down and you can no longer see the target, or allow you to unlock and rotate the periscope to look around while still tracking the target. Advisable to click the red button to deactivate the PK before firing, then get a new lock on the target so the data is more accurate. And don't forget to open the torpedo tube outer doors at least five seconds before firing, otherwise it will automatically open the outer doors then fire when the solution is outdated, practically guaranteeing a miss.