I'm going to reply, not because I can directly answer your question but because I feel like I may be able to help give you better insights into it.
I personally don't build things. I'm terrible at it and as a result it takes me a long time. Carpentry is a very weak skill and much of that has to do with never needing to develop it in my life. In general I've been forced to rely on others. Contrast that with mechanical or electrical work where I'm pretty good. I'm going to answer this from a mechanical perspective because I think it would relate best to your situation. I can personally, with the proper tools and enough time rebuild a motor. I've done it before. Today, I don't often change my own oil.
When hiring somebody to do work you don't just get the work you hired out for. You get the start of a relationship. You get to understand the quality of work you can expect. You get to understand the person doing the work, and their capabilities. They get to know you, and understand where you are coming from. In a good relationship, all of these things will help ensure an even better one moving forward.
When looking for somebody for a project I ALWAYS hire a higher skill set then myself. Often times much higher. This results in faster, cheaper, better work. Given how slow I am, how often I have to double and triple check things and how often I have to re-do some things as I've missed something. I often end up saving money or breaking even by hiring a professional for most things. That person often does a level of quality I simply can't be expected to do, at a pace that I can't compete with even if quality wasn't a concern.
It is a gamble to hire somebody to do work for you, but at some point you will hopefully scale to where that is required. By hiring early when it costs are low enough the mistakes are low. For example, hiring somebody to replace broken dry wall isn't going to cost you much if they screw it up. Hiring somebody to replace an entire kitchen could get very expensive if it was beyond their skill set.
Lets not forget, hiring is a skill. One you are going to end up developing if you scale. The earlier you start the better you will get at catching red flags. As previously mentioned if you can catch them earlier its cheaper.
All of these arguments are making the assumption that you are playing the long game. That you will want relationships in the future and that you plan on scaling your business, but not your personal laboring. It is important to factor all of that in.
Regarding an actual formula, here is one that I often use for most things. I figure out what I make hourly from my W2 job. I multiply that by 1.5. Then I factor in all of the hours, including travel time. I add in travel costs, lodging costs, etc. If I'm not able to work during the period of time then it is 2.5 my hourly rate. If this is a rush and I'm going to be working over 40 hours a week on this project I may add a penalty to that rate.
You can contrast that by 'paying yourself' to manage the project, hire the person, etc. I usually don't add those numbers in, however I've not had to have work done at those distances before.
