... The tone is heard by blowing across the "ff" hole opening from the side of the instrument. Each "ff" hole has its' own tone and must be shaped individually to adjust its tone. A larger open area (hole) equals a higher tone. Obviously the shape can be enlarged easier than made smaller so go about this slowly. Also remember that the shape of the "ff" holes is critical to the pleasing appearance of the instrument so they must be reshaped in such a way as to keep the overall appearance of the sound holes intact.
			... I have a whole series of special sanding sticks with convex and concave shapes to help make these adjustments while maintaining the attractive "ff" hole shape. When complete, you may want to color the inside wood surfaces with an oil paint such as 'burnt umber'. I paint it on with a small oil paint brush (after the instrument has been varnished), getting a smooth solid coating. Do not worry about getting any on the top surface of your already varnished instrument, because any that gets on the the top surface is simply wiped off with a paper towel. This leaves a pleasing rich dark tone on the exposed side wood.
	... What about the situation where the "ff" hole tone is too high already? Well, 300 HZ is another of those pleasing tones Mr. Ennemoser discovered and although this is slightly above D it might be a reasonable alternative. Another way is to build up the sides of the "ff" holes by carefully applying layers of clear varnish to just the edges. Each layer of varnish will become about 0.001" when dry but the "ff" hole will become smaller in area and its' tone will increase. Patience and perseverance will win out!