Deposit and staged payment calculator
Why stage it
A job that runs for six weeks and is paid at the end is a job you are financing. The materials went on your account at the merchant in week one and the labour is yours every Friday. A deposit covers the materials and proves the customer is serious; a stage at a visible point, first fix or the units in, keeps you level; the balance on completion is what the customer holds until they are happy. The guide to staged payments goes through what to ask for and what a customer will accept.
Making the parts add up
Split £4,200 as 40, 40 and 20 per cent and the parts are £1,680, £1,680 and £840, which come to £4,200. Split £4,199.99 the same way and rounding each part to the penny leaves the three of them a penny short of the total, which an accounts department will query and a customer will notice. The calculator gives any odd pennies to the largest stages so the sum is always exact. The app does the same when it bills an estimate in stages, and shows each stage invoiced or not so the same one is never billed twice.
Deposits, kept sensible
A deposit is for what the job genuinely needs up front: the units on order, the materials on your account. Ask for more than that and a private customer reasonably wonders what it is for, and if the job falls through the argument over what you may keep is one you would rather not have. Say on the estimate what the deposit covers, and issue a proper invoice for each stage rather than a request for money, so every payment has a document behind it. An estimate that gets accepted puts the stages on the estimate itself, which is where a customer expects to see them.