Can engineers form regular LLC's in your state? Here, Accountants, lawyers, engineers, etc can not, only Limited partnerships or a restricted form of LLC that still leaves you liable for professional error.
So, check with your engineering association to find out what level of liabilty insurance they provide to small (single person) consultancies.
This is one of the reasons that I am not going to work part time as a consulting engineer, in retirement, but will choose something else.
AND, of course, price yourself to include a proportion of taxes, medical and liability insurances, if not marketing and home office.
Good luck!