Buying Home w/ Partner. Can Percent Ownership Be Defined Legally?

My partner and I are looking to buy our first house. They also recently came into an inheritance and we're wondering how to take our next steps in a fair and responsible way. We've been together for over a decade but, importantly, have no plans to get officially married. First here's some info about us:

Ages: mid and late thirties. Jobs: Professionals with defined pension. No kids. Not having kids. Debt: None

It's important to both of us to maintain financial independence and both feel that we contribute equally.

We're looking at a few options for how to approach this, but one would involve partner using a significant chunk of the inheritance as a downpayment to lower our mortgage amount and paid interest.

For example: Find a home within our means. Let's say $400,000. Partner puts $200,000 downpayment, leaving us with $200,000 mortgage. We contribute to the remainder equally. Figure out "ownership" proportionally, for example on a $400,000 home; partner owns approx. 75% of home and I own approx 25%.

Is it possible for a real estate or family lawyer to set up an agreement/contract defining the percent of the home each of us would own and define how it would be split up in the event of the dissolution of our partnership? I imagine there must be some way since it seems like friends buying property together is more and more common.

We've been together forever and are both kind, level-headed folks. We plan to spend the rest of our lives together; however, I'm a fairly practical/cautious person and don't want to leave room for crazy messiness in future. I'll be honest my partner coming into this money is a gift, but I'm also uncomfortable with them contributing so much more to the home. Making it "fair" is important to me.