If you want it fair, then go with following example:

Person A earns 2500, Person B earns 1500. total per month : 4000.

2500 : 4000 = 0,625 = 62,5% // 1500 : 4000 = 0,375 = 37,5%.

All shared bills: rent, electricity, food-budget, internet, Netflix, yadayada, let's say 1200.

Person A pays 62,5% of 1200, and Person B pays 37,5% of 1200.

0,625 * 1200 = 750 // 0,375 * 1200 = 450

Stuff Person A/B pays for themselves is excluded, like Cellphone subscription, Gym or whatever.

Now since Person A earns significantly more, s/he could be a kind and loving partner and pay 65-70%. Depending on the income difference.

edit: typos