I'm trying to figure out when I'm due to ovulate on a calender. I'm confused about what to call my cycle length because the length has varied slightly for the last couple of months.

So, If my cycles lengths are
30 days - April,
27 days - May,

What do I count my cycle length as?

Is is counted as the last cycle length (27 days) or do I take an average (28-29 days)?

Please help