Your calculation of 25% depends on how the business is organized (I agree that it’s complicated to explain). But it is 20% of profits for sole proprietors or LLCs. It’s 25% for corporations and LLCs that elect to be taxed as corporations, but then I think then it’s only 25% of the employee’s salary.
From the Bogleheads wiki:
Employer Profit Sharing Contribution: An employer profit sharing contribution can be made up to 20% of net adjusted businesses profit. Net adjusted business profit is calculated by taking gross self-employment income and then subtracting business expenses and then subtracting 1/2 of the self-employment tax.
At least that’s my read. I’ve never done one, so maybe I’m missing something.
Also, I disagree with you that the limit is 35K. I believe you could contribute $18K through your employer (employee contribution limit) and then have your completely unrelated side business contribute $53K into your solo 401(k) (assuming you meet the above profit test). Can you explain why you believe it’s a max $35K limit?