Hmmn, kailangan pag-aralan talaga ang fertility cycle. Pag-mali ang information mo, baka di pala sa iyo yung nabuo, hahaha.
Obsolete na ang Calendar Method. Ang nakakahawig nito na modern Natural Family Planning ay ang Standard Days Method.
Ang pinaka madali intindihin sa lahat ay ang Two-Day Method (based sa Cervical Mucus aka Billings Method). The only catch is that you need to know if she has been secreting a slippery type of cervical mucus (which is critical for sperm survival and fertilization).
I think the best way to know if she's been secreting mucus is to watch out and notice if she's wearing a panty shield/pad. Kung meron watch out, malamang fertile siya. O kaya naman, notice mo kung paghubad mo ng panty niya ay kung may buo buo na parang egg white sa opening or pubic hair niya.
Very important ang mucus. Nakamamatay...ng sperm ang vagina, dahil sa high acidity nito. Yung slippery type na cervical mucus (egg white) ang sinasakyan (conveyor) ng sperms kaya protected ito hanggang ma-meet nila ang egg.
You guys are doing it the wrong way..
Why are you counting after the last day of mens? Day1 is the day her mens begin and the last day is the day before her next period begins..
If you want to avoid pregnancy then the thing you should be concerned about is her time of ovulation.. Ovulation usually takes place 14 (+/- 2 day) before her next period.
Lets assume na 28 days ang cycle ng GF mo.. She'll probably ovulate at days 12-16.. Sperm can live inside a woman for up too 5 days and her egg can only last for 24hrs after ovulation.. so if she ovulates at days 12-16 then you shoudnt have sex on days 8-18.. Theoretically she's infertile at Days 1-7 and days 19-28
IF you want to know more.. Google the Standard Days Method.. Its a system developed by Gorgretown university if Im not mistaken and is said to be 95% effective on woman that have a regular period thats within 26-32 days.