When I calculate the affordability of ACA, do I consider the tax credit I may have been offered or the upfront cost of the lowest bronze plan at face value?

Consider the bronze plan after the credit.  This is from the Form 8965 instructions:

"Determining an individual's required contribution— Individuals not eligible for coverage under an employer plan. If you or another member of your tax household can't purchase coverage under an employer plan, the individual's required contribution is based on the premium for the lowest cost bronze plan available through the Marketplace minus the maximum premium tax credit that you could have claimed if the individuals had enrolled in this plan. For this purpose, use the lowest cost bronze plan available through the Marketplace that covers everyone in your tax household: For whom a personal exemption deduction is claimed on your tax return, Who isn't eligible for employer coverage, and Who doesn't qualify for another coverage exemption. For information on the lowest cost bronze plan you could have purchased for your tax household, visit www.HealthCare.gov/tax-tool or contact the Marketplace serving your area. Subtract from the premium the maximum premium tax credit that you could have claimed if these individuals had enrolled in that plan. You can claim the exemption for unaffordable coverage for the individual if the result is more than 8.13% of your household income."