What are the best methods for calculating lift during different stages of flight?
As we have learned here at Avation Stack Exchange through many writings: thrust or power variation is critical in take off, climb, and cruise flight.
In straight line steady state flight (at the same speed) lift only equals weight in level flight, in climb or descent it is equal to the cosine climb or descent angle x weight.  In a climb the engine must provide the remainder of vertical lift, in a descent drag reduces vertical lift requirement.
Plugging in some numbers for cosine, we see lift is close to weight until climb or descent angles exceed around 20 degrees. (Cosine 10 degrees = 0.98 Cosine 20 degrees = 0.94)
Lift may exceed weight during maneuvering, known as &quot;G's&quot;.
This involves curving flight.  Every plane has a G limit, even models.  But in straight line flight, under most conditions, lift produced is fairly close to the weight of the plane, and is governed by airfoil type, wing area, air density, speed, and angle of attack.