Buy-in as co-founder at early-stage company

I am looking into a deal with a friend. She started a business (B2C & B2B with premium household goods) around 3 years ago, and built it to a business with around $350k in revenue, and $140k in-profit (she is not paying herself a salary). Projections are to increase to $570k in revenues and $260k in-profit (still without paying herself a salary) next year. She is very talented when it comes to design (which she would focus on) and I would focus on the operational/financial part of the business. I can see a likely future where a) we grow the existing business (from what I have seen & heard from customers, she really has product market-fit) by pushing more locally and internationally, and b) we venture into adjacent categories.

If I join (and probably "buy-in" myself), it's clear to me that I only want to do this if we are 50/50 in the long run. Any thoughts on a) how I should think about valuation/how much I should pay to buy-in, and b) how I can structure the deal to get to that 50/50 structure (either from the beginning or over time), while valuing all the hard work she has (very successfully) put in the business so far?

Any ideas/thoughts appreciated!