How to fairly share equity with a possible split looming.

[Steven Kotik]

I've tried to figure this out and I don't have a good idea of how this might work. I'm thinking of buying a property with my significant other and I can't help but think we might end up splitting in the future. If we purchased a place and I put 90% of the down payment and my s/o did the other 10% and we split the mortage, property, ins and expenses. It would be easy if we each put in equal parts down payment but that won't be possible. 

For example if we purchased a $500k property, she put $10k down, I put $90 down, monthly cost is $3K including all utilities and we split that equally. X number of years down the road property is worth $550k and we decide to part way - I don't want to sell and I want to keep it. What would be fair? 

If we did sell then commissions and any potential selling cost could erase any gains... if the value rose to $650K then what would be fair?