819,04 => 100 %
21,52 => p % => $${\mathtt{p}} = {\frac{{\mathtt{21.52}}{\mathtt{\,\times\,}}{\mathtt{100}}}{{\mathtt{819.04}}}} \Rightarrow {\mathtt{p}} = {\mathtt{2.627\: \!466\: \!302\: \!012\: \!111\: \!7}}$$ [ % ]
Probe: $${\mathtt{819.04}}{\mathtt{\,\times\,}}{\mathtt{2.627}}\% = {\mathtt{21.516\: \!180\: \!8}}$$
$${\mathtt{x}} = {\frac{{\mathtt{0.026\: \!27}}}{{\mathtt{1}}}} = {\frac{{\mathtt{2.627}}}{{\mathtt{100}}}} = {\mathtt{2.627}}\%$$