Scaling Third Axis in Chart

i used a slightly different method to MrShorty to determine the length of the horizontal line
in J8 i put 0.5 and in J9 use formula

Formula:

=COUNTA(D8:D42)+COUNTBLANK(D8:D42)+0.5

**even though your table finishes at row 41 i used row 42 to match your data range in your chart

everything else i would have done the same