Also, how long is your investment horizon? Is it really that important to you to project out to 30 years or is 3λ years sufficient along with a terminal value that represents the expected NPV beyond 5 years? Usually this latter approach works best and looks the most credible to potential investors. There are numerous ways to calculate terminal value including multiples, current market values projected forward, and round guesstimates. Obviously these decisions are affected by your personal preference and the type of investment for which you're calculating present value.
Customer Reminders: This is clever. Excel can be programmed to send an Email when a cell gets beyond a certain limit __ so if you're doing oil changes that should be done every few months, just collect the customer's Email address and have Excel use it to send them a helpful Email a few months after they had their oil changed last! The powers that Excel can offer your auto shop stretch so far beyond merely watching the profit margins that if you're not using it for more advanced functions, you're losing out. Stretch it's wings! Set some automatic alarms for your inventory and some automatic Emails for your customers, and see what Excel can do for you!