Ovulation testing

I am new to ovulation testing but we’ve been TTC for 4 months with no luck and I’m 41 so the pressure is on! I’m excited to report that in 3 days I already know I must be getting ready to ovulate because my line is getting darker but it’s tricky in telling which line is actually darker, which means 24-48 hrs?? Big difference when you don’t want to use up all the swimmers if it’s still a day or two or three away. Does anyone have any advise or expertise on this? Thanks in advance 😁