Projecting disk space usage

If the disk usage has gotten to 80GB with 8 years of 1,000 employees.

Would it be correct to say that to add 6,000 more employees, while at the same time recording payroll history of the past 8 years, one would do the following multiplication:

80GB * 7 = 560GB of disk space used after addition ??

I am needing to forecast the disk usage prior to converting and bringing the 6,000 wmployees into the Navision payroll system.

Any ideas? From what I have read from some users, it would seem that there may be too many indexes against the payroll database.

Thanks