partial R2 in multivariate linear regression

Hi there!

I´m performing multivariable linear regression models with SPSS. I would like to know how can I calculate partial R2 for the independant variables, specially for a group of them, i.e. to calculate partial r2 in % for smoking habit + alcohol consumption.

I´ve read that some people calculate partial R2 for the variables X and Y calculating total R2 in a model containing all the variables except X, and substracting it to the model with all the variables. Would it be OK?

Thanks a lot!