The function " Define rss" Is not summing up the equation correctly. It was working before I changed the scale of the Plot3D margins, but I really can't see why its not working. Any help would be amazing.

t = Import[
   "http://www.doc.gold.ac.uk//~ma302jh//RegressionExample.xls"];
tp = t[[1]];
t1 = Table[tp[[i]], {i, 2, Length[tp]}];
xcol = Table[t1[[i]][[1]], {i, 1, Length[t1]}];
ycol = Table[t1[[i]][[2]], {i, 1, Length[t1]}];
zcol = Table[t1[[i]][[3]], {i, 1, Length[t1]}];

Clear[DefineRSS];
DefineRSS[a_?NumericQ, b_?NumericQ, 
  c_?NumericQ] := (rss = Total[(zcol - a*xcol + b*ycol + c)^2];
  (a*x) + (b*y) + c)

Manipulate[
 With[{plane = DefineRSS[a, b, c]}, 
  Show[Plot3D[plane, {x, -10, 1000}, {y, -10, 100}, 
    AxesLabel -> {X, Y, Z}, PlotLabel -> rss, 
    PlotStyle -> 
     Directive[Yellow, Specularity[White, 20], Opacity[0.5]]], 
   Graphics3D[{Red, PointSize[Medium], 
     Point[Transpose[{xcol, ycol, zcol}]], 
     Line[Transpose[{{xcol, ycol, zcol}, {xcol, ycol, 
         plane /. Thread[{x, y} -> {xcol, ycol}]}}, {2, 3, 1}]]}], 
   PlotRange -> {{0, 1000}, {0, 60}, {-100, 500}}]], {a, 0, 
  1}, {b, -1, 5}, {c, -60, 300}]