Dollar vs pecentage commissions

["Yarroll" <komin@xxxxxxxxxxxxxx>]

Hello List, 

Is there any way to calculate commissions as percentages rather than dollar values?

In this way it could be possible to test a strategy on data going back to eg 1920s. 

Dow was then in the 200-300 range, vs 10,000-11,000 it is today. Dollar commisions are completely useless in such a test. 

Thanks, 
Yarroll