Hi @Arthur Fuller II, I would stay away from percentages. When you use a percentage you are essentially saying that expenses are based on what a landlord charges for rent. In real life we know contractors don't base their prices on your rental income so it doesn't really make sense to calculate your expenses that way. For example: In your area carpeting might cost $2,000 and need to be changed every 7 years. If you charge $1,000 in rent per month that is $84,000 over 7 years. That means $2,000 is 2.4% of your income. If you charge $500 per month your carpeting expense isn't going to go down to $1,000. Carpeting will still cost $2,000 which means the expense is %4.8 of your income.
To answer your specific questions.
Cap Rate - I wouldn't worry about this number if the property is under 4 units. Cap Rate is normally used to calculate the value of a commercial property. For non-commercial properties value is determined mostly by comps. Cap Rate is still a useful metric for comparing different properties but personally I wouldn't worry too much about it.
Vacancy - I normally assume the property will be vacant 1 month per year. This is very conservative for my area but you never know. Vacancy is determined by how desirable your property is. How much demand is there for the units you are looking at? Is your target rent more or less than comparable units? Vacancy will be determined by how good of a deal it is to your tenants. For the first year I would give a significant amount of vacancy since you will most likely have rehab time and time to find those initial tenants after the rehab.
Repair and Maintenance - Think about how often things break and how much you think they will cost. How often will the toilet need to be replaced? How often will doors, plumbing, drywall, etc need to be repaired? Depends on the age of your property and how well you think the tenants will take care of it. I use between $75-$100 a month.
Cap Ex - You can make a spreadsheet of everything in the property that will eventually need to be replaced - Roof, driveway, carpeting, flooring, kitchen and bath remodels, etc. Once you write everything out assign a useful life and a cost of replacement so that you can calculate how much it will cost every month. This depends very much on your location, how much materials cost, and what contractors charge. Maybe contact local contractors and ask them for general numbers. This number is very important since they are huge expenses. You don't want to think you're cash flowing $200 a month when in reality cap ex is costing $300 more per month than you anticipated. Here is a spreadsheet I made.
Cap Ex Calculator
Income Increase - How much do you think you can increase the rent every year? How much increase will your tenants agree with?
Expense Increase - Maybe use an average rate of inflation. I use 2.5%
Hope this helps, good luck!!!