An answer about house surplus

I had the same question as in this post A question about house surplus. I found that post when I was googling for an answer. But I couldn't find any answer online. So I called Fidelity and got an answer. I would like to share the answer but that post is archived and can't be replied. So I'm opening a new post hoping it could help others with the same question.

The answer is simple. Describing in my own words: my account is fine as long as my core position has enough cash to cover the house call (i.e. cash_in_core_position >= -house_call). Another way is to just check whether I still have "Available to Trade Without Margin Impact" since is the sum of house surplus and cash in core position.