Subject: Finding the velocity with non-constant acceleration w/o calculus

I am teaching a high school physics class and want to do a lab on terminal 
velocity.  I am dropping aluminum foil objects and can find the average 
velocity but I want to know the final velocity so I can determine the terminal 
velocity and the terminal velocity constant for the shape I am using.  I am 
approximating by taking twice the average velocity.  Is there a better way?

Requested info:  I am dropping aluminum foil squares from about 15-20 feet and 
I think they will reach terminal velocity before they hit the ground.  This is 
a low-tech lab.  We have meter sticks and stop watches.  We will take several 
times as the object falls so we should be able to calculate the terminal 
velocity if the velocity is constant in two consecutive time periods.  From 
there I believe B=mg/VT^2.  Before the object reaches Vt I can find Vavg=d/t.  
Vf=2Vavg if Vi=0 is only strictly true for constant acceleration which isn’t 
the case here.  I was wondering if there was a simple way to approximate the 
Vf in this case.  Thanks!

Re: Finding the velocity with non-constant acceleration w/o calculus