How the checker works
bol gives four boundary prices per item: the highest price at which you still get 5, 4, 3 or 2 stars. If your price is above the 2-star boundary, you have 1 star and bol does not show the offer. The checker finds the highest level whose boundary still covers your price, and works out how far you need to drop for each level.
Enter your floor price as well and you see per level whether that star stays above your floor. A star below your floor is bought at a loss.
Where to find the boundary prices
The boundaries come from bol's Retailer API (price-star-boundaries) and are recalculated about four times a day. Not every item has them: bol calculates them for about 60% of the range. Offerboxer fetches them per item, so you do not have to copy them yourself.
Is the next star worth it?
According to bol, more stars mean more visibility, and from 3 stars bol can advertise your item in Google. But every star costs margin on every item sold. Often 3 or 4 stars with a healthy margin beats 5 stars without one. More background: price stars and the relevant market price.