We tried to work out exactly the same thing km. Not sure what DPO you are but we did day 10 and got a BFN, but DH made sure I remembered that it could just be too early.
I used Pregnosis. Virgin user for this brand
, you put the stick through a foam disc which floats on top of your pee in the container. Kinda like sailing a homemade boat! Anyway
, the packet said can detect pregnancy hormone levels as low as 25 IU/L, normally reached 7 to 10 days after conception. (so that's before implantation, for your calculations)
Hope this adds to your decision making.