Eeeek! All I can say to that is good luck and overestimate rather than under. That way when it comes out less they are pleasantly surprised rather than blaming you for blowing the budget! lol
If your travel agency does not offer all different sizes of tickets you can add more days, paying just the difference in gate prices, after you get to Disney, or even slightly after (up to 13 days) you start using the ticket.
(Promotional tickets, typically with combinations of features you cannot buy at the gate, generally cannot be expanded or have non-expiration added.)