Yep, the 25miu test should give you a reliable result on the day AF is due. Some girls can get a BFP earlier than the AF due date (I was one of the impatient ones). They dont recommend testing prior to AF due date as it can lead to disappointment or even heartache especially if you've been trying for some time.
Depending on how far away you're due to test, Lullaby Conceptions (google them) has some cheap 10miu tests which you can buy singley and I think they are only about $3 each I think (sorry, it has been a little while). They also have very fast delivery (sometimes next day if you buy them early enough in the day) and I found them to be very reliable.
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