Cycle lengths - confused 😁

Little Tog
My cycles have been pretty regular since I came off BC (Mirena Coil) for 5 cycles now. Been actively TTC for 3 cycles and only the last cycle did I use OPK's and do BBT
They have  been, 30, 27, 29 and 28 days with 3/4 days AF, this cycle it was only 25 days - Which I'm assuming means we ovulate at totally different CD each cycle? I had no idea that they could change each month, so I'm assuming that doing the OPK's and BBT is the only real way to get a definate day of ovulation? Sorry to sound so naive at 35 but I've never really looked into all this stuff until now, when it seems so hard to get pregnant! 😁