Need advice on banister replacement

[Mac]

I’ve done a fair bit of woodworking but I’m stumped and don’t want to ruin the custom balusters I built. Someone tore out the original banister of my 106 year old craftsman and I’m replacing it.

Newel post - made and installed - final trim pending baluster decision.
Balusters - made
Handrail ready to cut.

Problem. Treads aren’t square or level.

Possible solution. Cut each down at the bottom by the greatest shortage and cope to each tread. <<< This one stumps me. Not sure what to measure to since each is different.

Photo attached.