I have a question!

I have long been looking at charging more in the style of landscape architects for larger projects: ie, charging my fee as a percentage of total completion sum, rather than as a lump sum payable on completion of various stages of works. I think that for longer projects it gives more flexibility: you can just work away on everything that the client needs doing without constantly recalculating whether each bit of work is already paid for or included in a previous sum, and having to constantly send new invoices to the client for, I don't know, renegotiating a tender action or sourcing a particular stone finish or something. I'd much rather just accept all these works as my responsibility knowing I'm being paid for all of them from the start. And I think it actually pays more in the long run too!

I've just had a proposal for a large and probably quite long-term garden project accepted at 7.5% of total certificated completion sum. I've had a good deposit, but the question is this: at which trigger points should I require further payments? I'm obviously requiring payment in advance of the final contract sum, which I don't know yet, and the completion certificate should just make up the final difference - but I can't wait until the end of the project to suddenly get paid thousands, my cash flow won't stand that. So, at what points do I require payment, and how much do I require?

Has anyone had any exerience of clients trying to avoid payment on this principle, too? It would be relatively easy for a client to disguise the size of his payments to the contractor, thus reducing my fee. I'm only acting as the client's agent to the contractor, not as project manager, so I won't necessarily see all the bills. I think that trust between the designer and the client is a basic necessity but I can understand the temptation. Are there any tricks of the trade to avoid this? Is it just a matter of really good communication with the contractor?

Any thoughts and experiences gratefully received!

Mark