A percentage is defined as a ratio with a basis of 100 as total substance. Convert a percentage to decimal implies to divide the percentage in 100 because decimal form has as basis 1.
For the isotopic forms:
1H: 99.98% → As percent.
99.98% / 100 = 0.9998 → As decimal form.
2H: 0.0115% → As percent.
0.0115% / 100 = <em>0. 000115</em>→ As decimal form.