What would be a good assumption for the discount rate of a dry cleaning service?

Yes. 12 month NOI = $64K. The question is about whether somebody should invest in a dry cleaning service or not.

But my question is less about this particular problem and more general -

How do you pick discount rates for typical businesses/industries when doing quick back of the envelope DCF calculations and you don't want to get into WACC and CAPM and all that? Is there a guideline somewhere? Is there a rule of thumb for picking a reasonable rate for a particular business?

Why bother with a discount rate? Just calculate the IRR instead.

I prefer the IRR if comparing among a bunch of bolt-on opportunities.

If I were picking a single decision like which business to invest in, then I would just use a discount rate based on whatever the next best thing is that I could do with my money (with a floor of 10% discount rate). But more likely I would factor in the expected salary that I would take out and then just calculate the IRR on the remainder (return on incremental capital investment versus a salary that I could get somewhere else).

Of course that would all be a quick screen to see whether the business was even worth considering. Mostly other factors would drive the ultimate decision, including stress and work-life balance, upside and downside of the industry, how easy it would be to get out if I change mind, and most importantly if it is something that I would actually enjoy doing on a daily basis.