My ortho team's goal, and that of the outpatient surgery center, was 90 degrees at six weeks. And goals are "aspirational," not commandments.
PTs absolutely don't get to set our goals.
Lying on your back with a rolled towel under your knee and lifting the leg to straighten it is called a "short arc quad." It runs a real risk of irritating your new knee.
Also at your early stage, I wouldn't put a towel under my knee with any weight or pressure on it (basic blood clot prevention!) and I would stick to quad sets instead - they provide a good quad work out without irritating the joint.
When I *much later on* progressed to short arc quads, the roll wasn't a towel under my knee but a slightly larger roll under my lower thigh, so there was still no direct pressure on the back of my knee. And no ankle weights!
Lying down and moving your extended leg outward is an ok range of motion as long as it's done slowly and not done to where it's bothersome.
Lying down and moving your leg around in the air or doing the knee bend things while standing are a problem - in my opinion - because you're doing all these repetitions in a short period of time. If you're at home going about your life, it's natural to incorporate a couple here and there, and the knee says "ok." For many of us it's the concentrated repetition of exercises in a PT session that put us into the Over Did It Club causing pain, swelling, and setting us back