With 3' and 45" at the km you are at the limit, but also for you there is the phenomenon of the drift of the rhythms below the aerobic threshold and if you do the calculation as Canova you would have too slow rhythms.
Therefore the simplest way since the calculation of Canova applies to high levels is to calculate the rhythms on the current human limit which is 172"9826 at km and then translate the value on your level.
So for example I want to calculate 80%:
So first 80% of 172"9826, but as Canova does:
172.98 / 100 * 20 = 34.6 seconds.
than 172,98 + 34.6 = 207 "58.
Then we calculate your distance from the human limit, always on the marathon pace:
225-172,9826 = 52, 02 second.
So let's translate the human limit value to your level:
207.58 + 52.02 = 259.6 at km.
4'19 "6 at km or 6'55"4 at mile is your 80%.