How To Calculate Average Row Size Of A Record.

 I want to calculate average row size of a record. By based on this i want to add some more columns into an existing table.
 Here is my table structure
CREATE TABLE patient_procedure(
    proc_id int IDENTITY(1,1) CONSTRAINT proc_id_pri_key PRIMARY KEY,
    patient_id int NULL,
    surgeon_name varchar(40) NOT NULL,
    proc_name varchar(20) ,
    part_name varchar(30),
    wth_contrast int ,
    wthout_contrast int ,
    wth_wthout_contrast int,
    xray_part varchar(60),
    arth_area varchar(30),
    others varchar(30) ,
    cpt varchar(20) ,
    procedure_date smalldatetime NOT NULL,
    mraloperrun varchar(20),CONSTRAINT patientid_foreign_key FOREIGN KEY(patient_id)
    REFERENCES dbo.patient_information (Patient_id))
 
Now i got a requirement that i have to add two more procedures with different columns.The columns overall size is 195 bytes.
I can place those two procedures as seperate tables. I dont want to do that becuase of front end requirements.
Here the problem is when the user enters these two procedures information remaining fields will store the  null value. I know that when we store the null values into corresponding columns min of 1 byte will be occupied. Please suggest me that shall i include these columns into the above table. If i add these columns is performance will be decreased or not. 
Waiting for valuable suggestions.