Ovulation Test Question

Ashlee
A question regarding ovulation tests. I started testing this cycle, and I've decided to test twice a day, first usually around 12-1, and then a second time around 6-7 (however long I can hold it *haha). My question is, should the test lines be gradually getting darker over the length of my cycle? Or is it normal for the test lines to vary in darkness before a positive result? 
Any advice is greatly appreciated. Thanks in advance! :)