I do this calculation often, as I fly a lot of Y trips LHR-CDG and always aim for 20 TPs per trip.

It may be different on other shorthaul routes, but to/from CDG, a segment must cost around £190 to earn TPs. Hence I often book outward in the cheapest buckets for around £60, as most of the evening return flights are in high classes only (typical fares £190 or £202). It's a simple enough task on any one route to play with the available fares and chop & change as necessary until you get a combo that give you 20 pts.

Whether you follow others' advice and buy a cheapy in Y and then upgrade depends on how fussed you are about losing the extra points when you hit silver.