How to align an object to another object's center point

This is a follow up to another issue I had where buttons are not clickable when they are inside 2 views

the workaround is to align the buttons on the same y-axis as the view, but I seem to be having troubles getthis to work.

This code works:

buttton.center = {y: '48.3%'};

but the issue is it's out of alignment on 4" iPhone and above (but is perfect for 3.5")

this code does not work

button.center = {y: buttonParentView.center);

And I get an error...its probably just some kind of formatting issue but any help'd be great. thankyou.