Okay, so I’m currently going through the grammar study for くらい and two of the example sentences are as follows:
何時ぐらいに来ますか。
あと３０分ぐらいで行きます。
Now, the で／に distinction has always been a rough spot for me but I can’t figure out why one is で and the other is に in this example. My understanding is that where movement is concerned, it’s always either に or へ.
Imagine you are in the space-time continuum and you are arriving “at” a certain time in the continuum → に.
Imagine that on the journey that you are traveling “with” an extra 30 min in your hand → で.
Too nerdy? 
I’m afraid I’m still not getting it. My understanding is that に is always used when marking a destination, and in example #2, the sentence is basically "After about 30 minutes I will go TO [unspecified destination]. So it makes sense to me that the sentence should read あと30分ぐらいに行きます。And I just re-watched the relevant section of the Ammo with Misa video and she specifically says that に is ALWAYS used with verbs such as 行く, 来る, and 帰る. (Timestamp 25:50 of AT? IN? To? With? に (ni) VS で (de) Differences┃The Ultimate Guide - YouTube)
So why does the aforementioned example sentence seem to contradict that?
If it’s 2pm now and you’re going at 2:30pm, you have 30min of time to spare with you. So a more complete sentence would be:
あと３０分くらいで ２:３０に 行きます。
I will go in about (with/using) 30min at 2:30.
あと３０分くらいで トムさんと ２：３０に 行きます。
I will go in about (with/using) 30min with Tom at 2:30.
Note:
– “30min” is not your destination, 2:30pm is your destination.
– If you use 行く・来る・帰る・戻る not every word in the sentence uses に.
Does that make more sense?
I think that makes sense. So basically the に actually is in the example but its just not visible because 2:30に is implied through context. Is that the gist?
You got it! 
Thanks! I still have a feeling that に and で are going to be issues for me in the future, but hopefully I will overcome!
It’s one of the many things that as learners we find difficult. I was the same but eventually I just got it. (At least to a better sense then when I first did it).