No harm in placing your maximum bid now. Only you and Ebay will know how much that is and your bid will be increased automatically in units of £1 (usually) so as to keep you at the top, as others join the bidding, until they reach the limit you have set. Ebay will then tell you that you must either increase your upper limit or drop out.
Don't forget to tick the "Watch this item" box.
What are you bidding for? Mine, who will be ten in August, wants a Cornet (plus lessons of course)