If you “selling” a $120,000 ARV by doing all that work your budget shouldn’t just be a rough napkin budget.If I were you I would revisit the numbers and see what the real rehab budget should be, and then work with the $80k ARV and base your “asking price” off that.

Just make sure it’s worth it with a little back of the napkin math...

Back of the napkin calculation, I see as ~56 months before you make your first dime on it (then again, I don't know your financing plan).Can you snag it cheaper? 

Paul, That works out great for "back of the napkin" math.

Love the tech, but sanitary napkins and Lego and everything else your tenants and their kids regularly flush won't compost too well.

Keep in mind this percentage is a quick, back of the napkin calculation to do a quick “test” to see where approximately the deal sits.

The filter is typically 24" and septic pumping guys charge anywhere from $110 to $160. this filter keeps hairs, and sanitary napkins etc (items that not supposed to be in septic system) keeps from entering the tank.

The 70% rule is truly a guide for quick napkin math.

It seems that over an extended period of time, residents have been putting sanitary napkins and wipes down the drains.

You could just write on a napkin "I offer to buy this property at ____ for $20,000" - but I would strongly suggest listening to @Chris Muller and paying someone to have a contract drawn up as a template.