I have to agree with "jonese". I would think this entry in your budget would be set by past history of your business or if no past history exists, extract data from a similar business.
On another note, we should be talking about percent of gross, not net (profit). Maintenance of anything is an expense that is just part of the overall budget. It comes out before the profit is calculated. I suspect (unless this business is heavily related to equipment), that this maintenance expense is the least of your worries.
Reducing equipment maintenance is what I do. If you would be kind enough to contact me, I`ll be more than happy to discuss your scheduled and unscheduled maintenance practices.
Albert "AL" Rich, ADR Oil Company, www.ADRoil.com, tollfree 1-866 364-5489