In essence, if I understand you correctly, you are asking for a standard for romanisation of tone diacritics. The straightforward answer of course is that each system has its own standard: Hanyu Pinyin for standard Mandarin is different to traditional Yale for Cantonese, which is different to Jyutping for Cantonese, which is the same as Sidney Lau for Cantonese.
With many learners whose first encounter with tone as a concept is with stnadard Mandarin Chinese through the orthography provided by Hanyu Pinyin, the diacritics are virtually featural; it is as if each diacritic was designed to represent the pitch of each tone as pronounced in the standard way. Neat, right?
This also works to an extent with Yale in Cantonese (including if you consider the high falling tone distinct from the high level tone, which is a minority in the 21st century). However, what to do with the low tones, which happen to be the same contours? Just add "h" after the vowel (or syllabic nasal consonant) and you're done! Not quite as featural as Hanyu Pinyin, but systematic enough.
However, Peh-oe-ji and its descendants are not very intuitive it comes to the representation of the tones in its orthography of Hokkien. If we look at the tones of Xiamen (Amoy) Hokkien, for which Peh-oe-ji was originally developed, through a strictly synchronic phonetic view, we get (in descending pitch order):
- high level [including obstruent endings];
- high falling;
- (high) rising;
- mid level;
- low falling;
- low with obstruent endings.
So we have two level and one kind-of level tones, one rising tone, and two falling tones. However, Peh-oe-ji was not developed with this view of the tone system; instead, it allocated the tones according to the historical four tone 平上去入 system of Middle Chinese, broken down by 阴阳 register, with 阴 going first then 阳. This is also technically the same view (let's call it the "historical" view) as the tone numbers for Cantonese in Yale and Jyutping.
The allocation of the diacritics seems fairly random, although an explanation from WH Medhurst's pioneering 1832 dicionary on (what would later be Malaysian) Hokkien can be found. Essentially, his tone marks are very subjective, introspective descriptions, and not directly related to pitch.