#CJam, 70 bytes, #Upvotes - 0.7

CJam, 70 bytes, #Upvotes - 0.7

Block wise explanation:

{{_9%)A@9/#*}%}:M;
{             }:M;              "Define a function M which takes an input array of"
                                "indeces and calculates the Gematri number for them";
 {          }%                  "Run this code block for each element of the array";
  _9%)                          "Copy the number, take modulus by 9 and increment it";
      A@                        "Put 10 on stack, and rotate to get the number on top";
        9/                      "Integer divide the number by 9";
          #                     "Calculate 10 to the power the above quotient";
           *                    "Multiply the above result by modulus 9";

rcrc),\i>:QlS-_@&
rcrc                            "Read the first two characters, representing the lower"
                                "and upper end of the character list";
    ),                          "Increment the upper end and get a list of U to ASCII 0"
                                "characters where U is the upper limit";
      \i                        "Swap and convert the lower limit to its ASCII number";
        >:Q                     "Slice the character list to get our desired character"
                                "list range and store it in Q";
           lS-                  "Read the rest of the line as string and remove spaces";
              _@&               "Take a copy, get Q on top of stack and take"
                                "intersection with the input string. If the resulting"
                                "string is empty, then the third input was a number";
                 {...}{...}?    "First code block is for string input and second for"
                                "number input based on the above intersected string";

{Qf#M:+}
 Qf#                            "For each character of input string, calculate its"
                                "position in Q";
    M                           "Get the Gematri numbers for these inceces";
     :+                         "Sum them all to get the final Gematri number for the"
                                "input string"

{i{Q,,M{1$)<},)Q@,=@@-}h;}
 i                              "Convert the input number string to integer";
  {                   }h        "Run the code block till we get 0 on top of stack";
   Q,,M                         "Get the first length(Q) Gematri numbers";
       {1$)<},                  "Filter and take only which are less than input number";
              )                 "Pop the last number from the filtered array. This is"
                                "The maximum Gematri number that can be deducted";
               Q@               "Put Q on stack and rotate the remaining filtered array"
                                "to top of stack";
                 ,              "Calculate the length of that array, which is also the"
                                "index of the Gematri number used.";
                  =             "Get the corresponding character to that Gematri number";
                   @@-          "Put the number and Gematri number on top and subtract."
                                "The next loop runs on the above result now";
                        ;       "Pop the resedual 0 from stack. The remaining stack now"
                                "contains just the Gematri characters."