In-season value moves faster than draft value. These are the rules the Waiver Room applies to your league every week.
A 90-yard game from a rotational back is noise. A backfield where the starter left on a cart is a role change, and role changes are what FAAB is for. Spend up to 25-35% of your remaining budget on a player who inherits a full workload, and single digits on a spike-week receiver.
Budget left divided by weeks left is the wrong frame late in the season. Once you are inside the playoff race, a starter for the next six games is worth nearly your entire remaining budget, because unspent FAAB is worth nothing in week 17.
Two startable tight ends is a surplus only if the second one is startable for the team you are trading with. Price trades against each roster's actual starting lineup gap, not against a generic trade chart.