Tax liens just give you a percentage back % which usually ends in bid down wars (@Wanye knows more about it but the general gist is other people can bid down  against you).Its after a few years of consecutive missed taxes does the property goto Tax-deed sale with the county.If you want to participate in tax-deed auctions you need a bit of money (40k to start possibly more now - and you'll need a lawyer if you intend on selling the property - have to complete the quiet title process). 

Also, is 25% the standard or is there a lower percentage for refinancing.

Your CapEx and Repair percentages (3%) are too low.  

Also, since my cash on cash return is not stellar, should I just pick my percentage of return and just work backwards to figure out my max offer?

You can either calculate times and percentages in your head, deal with people, and KNOW WHEN TO SHUT UP AND SIGN or you can't.Put down the books and start knocking on doors is my advice.all cash

What percentage, under the estimated value of the home, does the mortgage need to be to do these deals?

I've heard percentages as low as 33%.

Mike, When doing a subject-to I don't look at percentages, I care about equity.

Due to transfer taxes (a percentage of the amount for which the property sold which is paid to the government), the agent wants the price as low as possible if he has to buy it for the bank; thus keeping the taxes down.

If rents are higher, and you still want that $100, you can have a lower percentage.