My lines were very feint for first few tests but gradually got stronger, then really dark after about 100 or so!!! I think generally they say if there's a line in the right box at all then its detected the pregnancy hormone, but I think you need to do more tests. Do them 1st thing in the morning too. I dont really know about the round/square window thing cos I never used Boots tests and they all ask you to look for different symbols in different shaped boxes, but presumably one is to confirm its worked and the other is to indicate +/-? I'd say it sounds good if theres a line in them both!!
Good luck, hope its good news x