D66 means rolling two six sided dice, and labeling one of them to be read as the tens digit, and the other one as the ones digit. Such roll therefore yields a score between 11 and 66. This is very similar to the way you handle a percentile die roll (or d%) which is done using two ten sided dice.
If you want to simulate a percentile dice roll you can use the following table to get ballpark scores to beat or roll under (both inclusive):
| Percentage Chance | D66 Score to Roll Under (inclusive) | D66 Score to Beat (inclusive) |
|---|---|---|
| 5% | 12- | 65+ |
| ~15% (16%) | 16- | 61+ |
| ~20% (19%) | 21- | 56+ |
| 25% | 23- | 54+ |
| 30% | 25- | 55+ |
| ~40% (41%) | 33- | 44+ |
| 50% | 36- | 41+ |
| ~60% (61%) | 42- | 35+ |
| ~70% (69%) | 51- | 26+ |
| 80% | 55- | 22+ |
| ~90% (91%) | 63- | 14+ |
| ~95% (94%) | 64- | 13+ |
| ~99% (97%) | 65- | 12+ |
Note that not every percentage can be simulated this way, as d66 is simply not as granular as d%. Some percentages in the table above are given as approximations with the true odds given in parentheses afterwards.