Now, I'm utter bullocks at poetry, but I am learning Japanese. So allow me to (try and help by) describing how Japanese syllables work, and what Onji are meant to describe.
In Japanese words are spoken in a vary per-syllable fashion, with grammar sometimes being the only thing used to define the bounds between words/sentences. Along with this, Japanese words can be composed of syllables that have been prolonged to take up roughly two normal syllables worth of time.
Example, Tokyo(ときょ) is really written Toukyou(とうきょう), with the 'u's representing a prolonging of each syllable. Thus the word takes up four On rather then two, as Japanese speakers will take into account the proportional length of each syllable, not just how many there are.
Secondly, Japanese syllables usually go consonant, vowel, unless it's a lone vowel or a lone 'n' sound. However, words like Nippon(にっぽん) can copy the consonant sound of a syllable and put it at the end of the one behind it, thus prolonging it and adding another On to the word.
To-u-kyo-u takes up four On by prolonging each syllable's vowel sound.
Ni-p-po-n takes up four On by having consonants cap each syllable, thus prolonging them.
Se-n-pa-i like the other two, takes up four On despite being two English syllables, as 'ai' is pronounced as one but would have it's pronunciation drawn out.
So ya, basically, I'd use English syllables. Haiku are uniquely Japanese in how they work, as Japanese has a focus on the length of each syllable in a way that English simply doesn't.
Hope this helps, or at least was informative! :)
This is a bad, yet still proper 5-7-5 Haiku written in Japanese. Notice how it doesn't read like one in English:
when it's new,
it's a good Haiku.)