ok yeah makes sense thank you. Needed to use the val paramater to then modify. This is what I ended up using for anyone else interested:
.to('.full-screen-menu__ball', {
left: '-50',
ease: "power4.out",
duration: 0.65,
modifiers: {
left : function(val){
var newVal = parseInt(val);
if($(window).width() < 920){
newVal = newVal * 1.4;
}
if($(window).width() < 768){
newVal = newVal * 1.8;
}
return newVal + 'vh';
}
}
}, 'ball-moving')