Cool than you are not 'holding' the spot but rather 'pre~booking it' ... I would just make sure that you make sure the start date is 'tentative' because if your current clients have a job loss, transfer or something that changes plans on their end and the spot came open in April or something than you have the dilemma of do YOU suck up the lost income from April to September OR do you ask them to start paying a holding fee or whatever compromise seems fair to you!