A cone is inverted and filled with water to 3/4 of its height.  What percent of the cone's volume is filled with water?  Express your answer as a decimal to the nearest ten-thousandth.  (You should enter 10.0000 for $10\%$ instead of 0.1000.)
Let the cone have height $h$ and radius $r$, so its volume is \[\frac{1}{3}\pi r^2h.\]When the cone is filled with water, the amount of water in the cone forms a smaller cone that is similar to the original cone.  This smaller cone has height $\frac{3}{4}h$, and by similar triangles, radius $\frac{3}{4}r$.  So, the smaller cone has volume \[\frac{1}{3}\pi \left(\frac{3}{4}r\right)^2 \left(\frac{3}{4}h\right) = \frac{1}{3}\pi \cdot \frac{3^3}{4^3} r^2h.\]Hence the ratio of the volume of the water-filled cone to the original cone is \[\frac{3^3}{4^3}=\frac{27}{64}=0.421875,\]which, as a percentage, is $\boxed{42.1875}\%$.