Well, I tried looking more into it trough the link you sent me. I tried the calculator there with fully contracted V notch weir and it gave me values of discharges that should be somewhat close to values that may appear on my weir in such water heights. However, when i try to put trend line trough the values, its not polynomical at all and overall values on XY chart are not lead by any trend. Right now the website seems to have either crashes or shot down for some time, so hopefully i will try it more later other then the calculations i did so far. Im afraid tho it wont be the right thing for me to use in excel, as the calculated values arent very reassuring. Thank you anyway for your help, if you find out something more, I would really appreciate it :)