I think the formula is as follows: 1(one) lumen falling on 1(one) square foot of space is equal to 1(one) footcandle. Or lumens/sq. ft. = fc
Since you did not give the area that you want to light up, change the formula around to read: lumens /required fc = sq ft covered. or 25000/20 = 1250 sq. ft. You can see that if you want a higher fc rating the area will decrease. 25000/50 = 500 sq. ft.
There are alot of other items to consider, initial lumens, dust, height, angel of the light to the surface, etc..
I believe Philips has some information on this, check their web site at www.lighting.philips.com/nam