In the U.S. the word is pronounced in at least two different ways, even among people working in the high tech industry where this word is most frequently invoked. Both pronunciations you mentioned are both possible and enjoy similar popularity.
Merriam Webster includes three different pronunciations.
In the U.S. you can ...
The A always sounds like an O. I never hear it pronounced like an A.
It's the U that you hear disappearing in regular everyday speech. Sometimes it's briefly pronounced, but often not at all. You can see this described in this YouTube video. You might hear the U if someone specifically emphasizes that word in a sentence.
I'm a native English speaker from ...
Assuming ‘calculatable’ was intended: this will be accepted as valid but non-ideal spoken English by most hearers.
‘Calculable’ would be better, but there is a reason why your lecturer used the alternative.
There is a tendency to use -able as a suffix to completely unmodified verbs in spoken English.
Language is a statistical entity. Informal language ...
I think the advice you received means to pick one accent to learn and use yourself when speaking. The point of this is to simplify your learning. If you have to learn two different ways of saying everything then it will take you a lot longer to reach proficiency. There are many other places where English is spoken with other accents as well (Australia, ...
I am a visual learner (not a native English speaker) and IPA worked well for me.
Example: for the above "sit - site" https://tophonetics.com/ gives IPA
transcription /sɪt - saɪt/. It worked better for me than colors proposed in the other answer.
Warning: there are multiple ways way to transcribe English phonetics. Some dictionaries use their own, non-...