The two are linked, if she is in the habit of feeding to sleep she could well stay up 3 hrs screaming her head off for food/sleep so it's isn't as simple as just working out her A time but like I said, I'd try more like 1hr 30/40 as the 1hr 20 isn't getting her off even in the sling walking.
If you're ready to stop the feed to sleep prop then I'd just go for it - remember she can't change this, only you can.