Presumably there are three methods, +, -, and .
Presumably is a space because there is no index to apply to.
Is there any obvious sources which could be used as a cypher key?
Does 13 apply to the text from which the letter was acquired? Perhaps skipping over letters that are not in the same half of the alphabet, either by removing odds/evens or cutting it in half...
For example, if we're going the character-counting route, the fact that FFS has chosen a monospaced font would lead me to consider approaches that count all characters including spaces ahead of any based on word characters or word and punctuation characters only.
Also, anything in Proxxy's speech boxes should be taken very seriously. This is information that was too important / sensitive to be woven into the bulk of the ciphtertext. "I'll miss half the picture whether I think positively or negatively" - this has got to provoke curiosity in someone other than myself, for sure, right?
EDIT: As an example, and again if going the character-counting route, you might try counting from the beginning of the line on lines with a plus, but from the end of the line on lines with a minus, thereby discarding half the line each time (or half the picture) but a different one depending on sign. Not something I would've thought of without the speech bubble clue.
Someone try these:
+> with method1, -> with both method1 and 2, going from the left;
+> with method1, -> with both method1 and 2, going from the right;
+> with method1 from left, -> with both method1 and 2 from right;
+> with method1, -> with both method1 and 2, full line;
Any with all three methods, full line;
Any with all three methods, continuous;
+> from left, increment by count (# of letters/chars), -> from right, increment by count;
+> from left, increment by count, -> from right, decrement by count;
The above two, but increment by count TIMES the result of method1 and 2 (<13 = -count, >13 = +count)
The above three, but count increments with method1 and method2
time to do my geography homework...