Werner;
Thank you for your great comments. I have revised the weir and orifice section of the sheet as I found more accurate empirical formulations. As for the weir equation there appears to be no closed form solution for this, other than using elliptic integrals which are impractical and cannot be calculated by MathCad's symbolic engine, anyway. Mathematica gave me a solution but it was 2 pages of indecipherable math, at least to me.
Good working with you. I have some ideas for other challegers but that is enough for now.
Kind regards, Mark