I have zero experience on steel, but I am an electrical estimator and project manager for commercial buildings. Anything from small tenant finishes/buildings as you mentioned up to large schools, hospitals, hotels and manufacturing plants. We use estimating software that links to standard labor pricing. The major materials are sent our for quote and the smaller commodities type items are priced off of the weekly updated database. All items are installed in the estimate using the software which automatically assigns a labor hour or percent of labor value for each items or linear foot depending on how it is set up. We also add any misc. items, labor, rental equipment such as lifts, etc. and set the profits, tax in the software and adjust with multipliers to the difficulty of the project and phases.
Almost any type of trade whether electrical, plumbing, concrete, steel erection, etc. have national labor standard unit books which give the average labor per foot of steel, wire, conduit, etc. You could get one of the books and simply build an Excel spreadsheet to extend the labor values and other items. One of the names of such estimating labor hour guides is RM Means. Google it or a general search for steel erection labor units.
Add overhead and burden % to your labor values and also overhead, profit and tax to your materials/rentals. (any costs for running your business such as leases, rentals, insurance, benefits, gas, small tools, etc.)
Sorry that this is not specific to your trade and basically general but maybe it will help.
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