I am an amateur at this kind of thing, but this is how I see it.
Your first year running your own business, is for you to figure out just how you are going to do things, and among many other things, how to properly cost your jobs.
What works for others may not work for you.
I suggest you get yourself a spiral notebook and keep copious notes of everything you do, how much time it takes, and what funds you spend, miles driven, tools bought, etc.
Every night make a summary and put them into a spreadsheet.
Keep track of how much time and money you are spending on these little jobs that is not directly related to actually doing the work, and make sure you figure some way to get paid for the time and money spent.
I think you will find that you are underbidding these projects. That's OK. I would expect that upfront. You need to gain experience in running a business, and the only way to do that is to actually run one.
Over time you will develop a feel for what makes you money and what does not. You will also almost certainly discover the 80-20 rule. 80% of your profits come from 20% of your jobs. Your goal should be to eliminate the jobs that make you almost no money but take up a lot of your time.
It may sound like you are making a lot of money on a service call to fix a light switch where you charged a customer $50 for the work. In reality, you probably lose money on such a call, considering the time it took you to drive there and back, and what you could have been doing if you were not off on that call. I urge you to develop a hard nosed philosophy about those kind of jobs. Nothing wrong with doing service work. It can be lucrative, BUT make sure you are actually making money on the deal.
Don't expect to do any better than break even your first year. Very few businesses even do that well. Hopefully your personal finances will give you the time to break into the black before your capital runs dry.
Avoid buying lots of toys for the tax breaks. It is a losing proposition. You have to make a profit for the tax breaks (such as they are) to be worth anything, and you probably won't make any money your first year anyway.
In the end, running a business is about making a profit. You are guaranteed to fail if you ever forget that.