Port wine grading system, how does it work ?

Dear Sirs,


Can anyone please explain me or refer me someone that can explain me
very well the grading system of Port wine:

I want to know the follwing table score are given according to what
factor value ?

I mean the score are given how ? accoridng to what value of the left
hand side factor considered and listed in the below table ?

Grading Table  
                        
factors considered      minimum score   maximum score   
                        
 soil and climate                       
                        
location        -50     600     
                        
altitude        -900    240     
                        
slope of land   1       101     
                        
bedrock         -400    100     
                        
rough matter    0       80      
                        
exposure        -30     100     
                        
shelter         0       60      
                        
 agricultural                   
                        
yield   -900    120     
                        
vine stock      -300    420     
                        
planting density        -50     50      
                        
training system         -500    100     
                        
age     0       60      
                        

Dapendign on score result we get:
                
class   score   
                
 a       >1200  
                
b       between 1001 and 1200 points    
                
c       between 801 and 1000 points     
                
d       between 601 and 800 points      
                
e       between 401 and 600 points      
                
f       between 201 and 400 points      
                

Thanks,

Mario