Robert,
Just like Kim, I actually find the commissions as % of the spread value more useful than % of the profit. I have both of these on my spreadsheet that tracks trades, but I find the % of the spread value more useful. It makes it easier for me to see my profit target. (e.g. if I am targeting 10% net profit and the commissions are 1%, then my real exit target is 11%.)
I basically add the commission cost for both the entry and exit to the initial spread cost to find my break-even price point (for IB, I just use the 0.75/contract as the average cost.) Then I have values for various targets (5%, 10%, 15% etc.)
This way, I can also use the commission cost to choose among the various brokerage firms I currently have accounts with (with different commission rates.) e.g. for lower priced spreads and high number of contracts, I might choose a different brokerage firm to reduce my commission overhead.
So, the commission as % of the spread helps me take/plan an action even before I have a profit (and hence commission as % of profit doesn't exist yet.)