Why do we meaning me overthink things

[happyfingers]

Hi everyone
I found a cute but easy lace chervon pattern it clearly states coast on mult of 12 + 5 so I made a small swatch with a cast on of 17 worked wonderfull loved it then I decided to do three repeats to really see the pattern so what do I do cast on 51 stsand I'm just happily knitting away I look at it and say hummmmp what's wrong because if you have guessed it, I have a triangle and this big column then another triangle then I look at it and a light goes off I was suppose to cast on 41 sts not the 36 sts plus the 5 sts I added for each group of 12sts oh my goodness that why I love knitting I can strech my brain a bit because it surely needs it you all can imagine what math was like for me in school lol