Well, here's a possibly too simplistic of an answer..

If with full sha1 you get about 1 in 2^160 chance of collision, 
then by truncating one character you increase the chances of 
collision by 16 (all possible values of the truncated character)... 
which is 2^4.. So, if you truncate x characters you get 1 in 2^(160 - 4*x) 
chances of collision.. right?