While trying to find underlays to Lay, I found that the Prices as produced by the ratings are not directly compatible with the Prices of the horses on your live screen. That's because different rating systems produce different $ figures against a horse but this should not be compared directly with the $ on offer on a horse on Betfair say. What I should be doing is converting the Ratings $ prices to an order of favouritism as in 1,2,3,.... Then compare that order with the order of favouritism of the live SP $ prices.
For example say Horse # is rated at $12 and it's SP is $8. You think Underlay, I'm going to Lay it, but if you converted the Rated $ and SP $ prices to a 1,2,3... order you might find that Horse # is 6th favourite in both columns and therefore not an underlay. When doing this method we are looking for horses with opposing orders. Horse * in position 1 in the Rated $ column but in position 3 in the SP $ column is an Overlay. Conversely horse # in position 4 in the rating $ column but in position 1 in the SP $ column is an Underlay.
So take the prices you see in the Tele as only an indication of the ORDER of favouritism only! This might be common sense to some but maybe I've helped some newbie from losing some money?
Cheers RP