Maths help for Novice

chelsea -1.5 and blackburn +1.5 in English Premier League Soccer.
I figure Chelsea to average 2.0 goals.
I figure Blackburn to average 0.9 goals.
How do I compute the percentage chances of Chelsea covering and winning by 2 or more goals?
I have Stanford Wongs' book "sharp sports betting". I can work out the percentages if the margin is -0.5 or +0.5 using the 'poisson charts' in Stanfords book, but I can't figure it out for -1.5 or -2.5 etc.
I'm not even sure if this is an appropriate use of the Poisson charts.
Any help would be much appreciated.