Percentage of cells containing X?

I would like a formular to work out what percentage of cells in a range have a certain contents. the contents of the cells is preferably letters (eg 'H') as it makes assesing the information at a glance easier than numbers.

So i want a cell to show what percentage of a range contain H. If it helps the percentage value of each cell in each range is 0.45%

Cells in the range could have a variety of letters, but only one for each cell.

I'm Guessing it should start with 'countif', but my formula of =COUNTIF(B10:F19,"H") isn't adding up until after i re-open the file which is a problem as the user needs to be aware if the percentage goes too high as the information is added (I plan to use conditional formatting to change the background colour of the cell) is there a way to instantly calculate changes to the range, and automatically work out the percentage?

Any help on whether this is possible is much appreciated.