I agree with @Anderson_Geten that you start with your cost. I use https://www.fuzzwork.co.uk/blueprint/ to determine current Jita material pricing for the items I build. Steve also calculates invention and production cost. You then need to add market fees (brokerage and taxes) and a profit margin you are comfortable with.
At that point you look to see if the product can be sold profitably - is the price you calculated inside the market cycle for the product? If so you can build it, list at the calculated price and wait for the market to come to you. If current market pricing is significantly higher, you can list at market but adjusting your price is now very expensive - you don’t want to do it often!
Some T1 rigs are very profitable. For example: https://www.fuzzwork.co.uk/blueprint/?typeid=31716
I’ve been using that example for years - it still works!
Industry scales well across characters. Thus, with 3 characters on an account you have up to 33 manufacturing and 33 research slots for the cost of 1 subscription and the jobs run in the background - you don’t even need to be logged in. There is, obviously an investment in training but CCP has been offering MCT promotions several times a year that helps. The second and 3rd characters on the account can also do PI for additional income or, in my case as a T2 manufacturer I use the PI I harvest in production. Once all characters on the account are fully trained the subscription queue can be used for skill farming.
I don’t consider profit per slot - I’m rarely using them all and some of the slots I am using are for intermediate products like advanced components. If my wallet keeps getting fatter I figure I’m doing something right!