~16% increase in FOV
One way is to use Bob Atkins' calculator at--
http://www.bobatkins.com/photography/technical/field_of_view.html
(About middle of page)
Mothman's charts show the field of view for focal lengths from 17 to 600mm at distances from 5 to 90 feet.
5 - 30 feet:
http://www.pbase.com/mothman13/image/74511515
35 - 60 feet:
http://www.pbase.com/mothman13/image/74511517
65 - 90 feet:
http://www.pbase.com/mothman13/image/92899167
Mothman's tutorial image viewer--
On this page, you can select one of four focal lengths (300mm, 400mm, 500mm, 600mm), one of 4 distances (100 feet, 150 feet, 200 feet, 300 feet) and one of three birds (12" Blue Jay, 24" Mallard and an Osprey with 65" wingspan). Click the button and you'll see a picture showing what the full photo would look like with your selections (scaled to 800 x 533 to show on the page). Switch one or more parameters and you can make direct comparisons.
http://www.texasmothman.com/photography-tutorials/birding/birding.asp
Generally, cropping FOV is ~linear--
420-mm/500-mm = ~0.84
0.84 x 500-mm = 420-mm
~16% crop of 420-mm FOV to yield ~500-mm FOV
In my experience, > ~30% FOV cropping is when one starts to take a hit on IQ. It's almost always better to have the longer reach than to achieve the same FOV by cropping for comparable IQ lenses-- unless one is starting with a prime and comparing to a long end of a zoom.
Here's a Bigma image at 320-mm (480-mm EQ)
at 420-mm(630-mm EQ)
at 500-mm (750-mm Eq)
Keep in mind that adding a TC will always have an impact on IQ; less for a 1.4X than a 1.7X TC; less if on a great prime than on a zoom; Generally, a 1.7X TC is about the upper limit one would want to attempt -- IQ will ALWAYS be best without a TC