I am not familiar with the term DR, but I have solved puzzles that use various manipulations of words to get coordinates.
Things to consider - in your example - would you be expected deduce a single digit from the word CACHE? If so, then you would need to do additional operations on your sum of "20" to yield a single digit, or find another way to convert words into numbers (sometimes length of word, sometimes using just one of the letters (first, last, some sequence (first or the first clue, second for the second, etc.)). Look for patterns that might fit with some of the coordinates you know.
You could ask the cache owner what DR means (it would seem from your question, that is not the significant part of the puzzle), or look at more surrounding puzzles to see how it is used locally. I was able to see some info on the page of a cache close to the unknown you have already found that gives spells out what DR stands for, as well as giving a link to a tool that will help. To help find just unknowns, there is a link on each cache page under the hints section that lets you find nearby caches of this type.