Should I use "PreSeed" on my last fertile day?

Today is my last day for my fertile week my boyfriend and I have been trying to conceive for sometime now. He recently bought "preseed" and we're planning on using it today since I came home from a trip. My question is should we wait until my next fertile week or try to use it on the last day? I know that my egg can live up to three days and males sperm can live up to 3-5 days.... On glow obviously my chances are slim but should we try anyway? 
We did have sex before I left as well but one to make sure this time I concieve....  What do you think? 

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