Clarification on "HBP" Tax Liens in Baltimore

[Scott Taylor]

I've been looking at tax liens in Baltimore, but don't quite understand the "HBP" - the "High Bid Premium".  Here's a quote from the FAQ:

"In Baltimore City the high-bid premium shall be 20% of the amount by which the highest bid exceeds the greater of a.) the lien amount; or b.) 40% of the property’s full cash value (note the full cash value is the assessed value). The High Bid Premium is to be paid on the date of the sale along with all taxes and other municipal liens, interest and penalties, and all costs incurred in the Tax Lien Certificate Sale.

Questions:

1. Is the "date of the sale" the date of the tax lien sale or the sale of the property (if the property owner defaults in paying their payments)?  As a concrete example, if the property is assessed at $100,000, the lien is $1,000, and I bid $1,000 - and assuming I'm the only (and highest) bidder - what do I owe the city of baltimore when the auction closes?  $1,000, or (40% * 20% = 8%) of the cash value of the house ($100,000 * 0.08 = $8000)?  If the latter, am I only making 18% on the $1000 I've invested in the lein, and not the $8000 that I've paid the state?

2. What % of properties can be acquired in the auction (not OTC) by offering just the lien amount by bidding?  Looking at the CSV from a previous year it looks like many of the occupied houses were acquired by the city, meaning that no one bid on them (?!)