I would suggest plotting at least half a dozen marks and then averaging the course between them. The more marks you can create, and more time you can observe, the more accurate your plot will be. If I have the luxury I will often continue to plot during the entire overhauling maneuver, resulting in dozens of marks which leave me with a very clear and accurate course. Also, don't be afraid to take them at quicker intervals than 3:15, just make sure you do get at least one or two 3:15 checks to get speed. It's worth noting you can take shorter speed observations as well as long as you multiply your result accordingly, ie making roughly 1 minute marks, then times your result by 3. Once you get the speed though, just keep observing and marking as long as possible to hone in their course and therefore ultimately AOB and range as well. Then make your intercept and you know what comes next
As an aside, it's well worth learning some basic periscope methods for obtaining speed, range and AOB, good for last second adjustments when you are very close to the attack. Also if you make the jump to SH5 and Wolves of Steel (highly recommended when you are ready for another "realism" bump), with Real Navigation you will probably more and more rely on visual observations vs plots (this due to the fact that you will need to keep track of your boat as well as their boat if you choose to plot, not impossible but perhaps impractical in many situations). Overall, my personal favorite method is the RAOBF wheel.