I'll elaborate a bit on Barrie's point, which is correct, if disappointing.
The problem is that English spelling was not designed for Modern English. It was designed for Middle English, a very different language. When Middle English changed its pronunciation to become Modern English, English spelling did not change. Furthermore, English borrowed many thousands of words from other languages, which were of course pronounced differently, and spelled differently still.
The result is that one has to choose between two strategies in learning English words, however they are spelled -- this is not a problem confined to the letter I -- or else figure out some way to mix them.
Either you can actually learn the historical rules about pronunciation and learn to distinguish the different kinds of word each rule applies to -- which amounts to learning some basic linguistics,
Or you can do as Barrie suggested, and memorize 2 things about every word you learn -- (1) how it's spelled and (2) how it's pronounced (Kenyon and Knott is your friend here) -- and just ignore the possible but treacherous correspondences you might suspect between Middle English or foreign spellings and Modern English pronunciations.
The second option amounts to giving up all hope of making sense of English spelling. Most native English speakers do this, which is simpler for them, since they already know the pronunciation.
Since Anglophone education systems don't teach anything about English language, they never learn any different, and many still believe there should be a simple rule for pronouncing every letter.
It's not entirely clear what you are interested in.
With personal names (first and last name/surname), you pronounce it the way that person does. That's why you have to listen carefully when they introduce themselves. There's a lot of variation there. For example, I've met women whose name is TAmara (on the 1st syllable) and whose name is taMAra (on the 2nd syllable).
As for other proper nouns, such as place names and what not, there are dictionaries for that. The general rule is to try to get as close as possible to the original (foreign) pronunciation, applying rules of English phonetics.
Best Answer
English speakers commonly leave out unaccented syllables in certain words. "Double you" is one of them, at least in the U.S. Look at Merriam-Webster's pronunciation of this word. They give double-you /ˈdʌbəlˌju/ , dubba-you /ˈdʌbəˌju/ , dub-you /ˈdʌb.ju/, or dub-ya /ˈdʌb.jə/. The speaker in the video is saying dub-you D forty and not double D forty. All these pronunciations are correct.
There are many other words where we leave out unaccented syllables, particularly when we're speaking fast. A few examples: comfortable also gets pronounced comfterble, probably is also pronounced probly, and interesting can be pronounced intresting.