How to design a perfect sleep schedule?
If you have rigid brackets imposed on your schedule, and still want to be biphasic, you should try to always take a nap 8 hours from waking. If waking is not natural, this number can shift to later hours. You should go to sleep at a time that makes sure that your ideal sleep length results in waking at the desired time (e.g. for work). If you cannot fall asleep, it usually means you went to sleep too early (assuming no other interference).
For details see: Science of sleep.
In your case, if you wake up at 6 am, your optimum nap should come at 13:30-14:00. In that sense, your schedule seems perfect. You should rather not plan 20 minutes of napping. Just to go sleep at 13:00 and try to get as much sleep as possible. If you try to wake up with alarm, the value of the nap will be reduced.
Follow up questions
Q: What if I find it difficult to fall asleep for a nap.
A: This means your night time sleep was excellent, or your napping conditions are imperfect (e.g. stress, noise, discomfort, etc.)
Q: What if I need to go to sleep later on a single night. Should I use alarm or just sleep longer? What time should I nap?
A: Such departures will result in confusion and it will be hard to make a good prediction. Ideally you should wake up at the same time, and use no alarm clock. Those two are irreconcilable. If you wake up naturally with a shift, nap could be your first time to attempt to shift back to your rhythm. There is no good solution apart from avoiding missing your bedtime. If you did not have time constraints, you could sleep as much as you need, wake up later, and try to recover your original sleep by gradual shifts in time backwards. Those are very hard.
Q: What if I need an all-nighter? What time should I go to sleep next day?
A: You have to go to sleep late enough to make sure you do not fall into total asynchrony. For most people this would probably mean going to sleep 2-6 hours earlier than usual. In your case, you would probably sleep 8-12 hours to recover. You would need to test it by trial and error. Hopefully, you will not do it often or at all.
Q: I am 19 years old. Does it change your recommendation?
A: At 19, you need more sleep, and your system is probably "undamaged", which should give you a good start into many years of good sleep assuming you do not experiment too much, and don't play with your sleep schedule.