my .2s on the subject
most offen people want to changlen using blast rules saying it will be to hard to judge the distances
my sujestion for most blast rules(grenades/mortars/what ever) is to use the med the group has (15-20' in most places i have been). if you can't instantly judge you are sub med you shouldn't really be playing
what we have done is a "hard cover" rule, ie unless you are behind hard cover (sand bag bunker, earth works, ect) you are "hit".. trees and the like don't proterct you, call it the overpresure taking you out, yes its done for game play simpulfaction but realy its the same as being "hit" in the pinkly/camel back and caling your self out.
as with most things AS the simpler you keep the rules the smoother and usaly more fun it is