[Alborz Riazi]

Thanks a million, this helped a lot.
btw I had to tweak the code just a tad to compensate for the decimal values in my property values. it couldn’t find the frame

var compLength = thisComp.duration / thisComp.frameDuration;
var property = thisComp.layer("A").text.animator("X").selector("Y").start;
var flag = false;
var wantedFrame;

for (var i = 0; i <= compLength; i++) {
    if (Math.round(property.valueAtTime(i * thisComp.frameDuration)) == 100)
    {flag = true; wantedFrame = i}

    if (flag == true) {break}
}

wantedFrame;

[Alborz Riazi]

Nearly two years after you posted this, it helped me loads. Thanks a million for the great expression.