VBA Code & Formula Interference

See attached spreadsheet.

Column Q CANNOT be greater than 50% of column P.

Column P should have an amount entered FIRST
If column P is changed, then column Q Should Clear.

So far this is working...except for the scenario where column P gets changed and column Q continues to be Less than or equal to 50% of P.

Column Q should change AT ANY TIME THAT that column P is changed.