Owner's duties.(1)To protect the physical health and safety of the ordinary renter, an owner:(a)may not rent the premises unless they are safe, sanitary, and fit for human occupancy; and(b)shall:(i)maintain common areas of the residential rental unit in a sanitary and safe condition;(ii)maintain electrical systems, plumbing, heating, and hot and cold water;(iii)maintain any air conditioning system in an operable condition;(iv)maintain other appliances and facilities as specifically contracted in the rental agreement; and(v)for buildings containing more than two residential rental units, provide and maintain appropriate receptacles for garbage and other waste and arrange for its removal, except to the extent that the renter and owner otherwise agree.(2)Except as otherwise provided in the rental agreement, an owner shall provide the renter at least 24 hours prior notice of the owner's entry into the renter's residential rental unit.(3)(a)Before an owner accepts an application fee or any other payment from a prospective renter, the owner shall disclose in writing to the prospective renter:(i)a good faith estimate of:(A)the rent amount; and(B)the amount of each fixed, non-rent expense that is part of the rental agreement;(ii)the type of each use-based, non-rent expense that is part of the rental agreement;(iii)the day on which the residential rental unit is scheduled to be available;(iv)the criteria that the owner will consider in determining the prospective renter's eligibility as a renter in the residential rental unit, including criteria related to the prospective renter's criminal history, credit, income, employment, or rental history; and(v)the requirements and process for the prospective renter to recover money the prospective renter pays in relation to the residential rental unit, as described in Subsection (4).

Napkin math has a refi with 80% LTV able to pull out just shy of 100k. 

In your experience, are there any ‘back of the napkin’ calculations or ‘rules of thumb’ that you follow or have found useful?

Some investors use calculators that you just plug these numbers into, some use the "back of the napkin" method, others like @Justin Elliott use formulas like the 1% rule, etc.

For MLS transactions, it'll be expected that you use the specific state's boilerplate realtor contracts.For true(1) off market, or seller-direct, type transactions, you can use what you want, inclusive of a bar napkin that contains the elements necessary for a legal contract (which may have some slight state variance if you're doing something super esoteric, but otherwise probably not).(1) Note that if it's being marketed to you, by definition it is not "off" market.

Plus you had to put down 25% or $400,000 down.Pull up a spreadsheet or get with buddy on a napkin and ballpark the calc.Then come back and adjust. 

Keep in mind that the 70% guideline is just a back of the napkin calculation, it is not the end all be all.

Given there's not enough information to quote or do anything like that, but back of the napkin, it sounds like you should be able to qualify. 

Which 5% on that is nearly nothing.I figure that for one to make some money you need massive scale purchasing certificates.Some napkin math: if I aim to make $3k this cycle I need need to buy about 100 of the $600 certificates, which means I need to bid on at least 3000 certificates.Is this what people do to make money on tax liens?

Back of napkin, I worry that you won't have much left over after expenses.