There is no set % when your small. Look around the lawn site and you see some with all new trucks and mowers with payments and some with old junkers and some frightening annual repair bills. Figuring out the useful life of trucks trailers and equipment is more delicate and difficult than some here preach. Some pop wheelies with the mowers and take the turns on trailers too fast. Or they think every time they find themselves at a stop sign or a light that they are in race car and need to reach 20 mph over the speed limit in the next 5 seconds or they are somehow not a man. The advice a guy like that gives you on useful life of your assets is going to differ massively from some one that treats his stuff like his children.
Iv found I needed to test and experiment in all aspects of equipment life to get my numbers that I now use. 2 cycle equipment was easy to get a handle of 2-3 years and its of no more use to me and that is info we use in buying new. Trucks are much trickier they can last over 20 years but they have a point when selling makes sense where they still have half or more of their value and are young enough to have thus far been trouble free which will be ending soon and you know it. Mowers were harder to Gage since we got them all knew and running them till they died seemed to be a wise idea looking back I know long feel that way. But that info was hard to learn and I cannot give it to you you need to experience it for your self to see the truth in it.
Your far to small to think in terms of % and need to think in terms of replacements and upgrades and of course expansions, and when those purchases need to be made. Once you have a schedule of new purchases you can figure out how long you have to save up and there by have your real number.