HELP! Different Cycle lengths when do I test for ovulation

My period cycles are so crazy! I'm not sure when I need to take my ovulation tests! I am taking the First Response ones. My cycles for the last 3 months have been 42,33, and 29. It says to take the average from the last 3 months but I have no clue how to do that when my cycles are weird like that. I took one this morning and I don't think it's positive but I'll see what you all think. Has anyone else had the same problem?