If your printer isn't too exotic you might want to start with a fresh config from the web configurator to save yourself from having to manually adapt it all. At the very least it would be worthwhile having a new config to compare against.
Is it a delta printer or a Cartesian? If it's a delta, see the wiki page on calibrating a delta printer. If it's a Cartesian or CoreXY, the endstop positions are assumed to be the same as the axis limits defined in M208 S0 and M208 S1.
To put the nozzle at X0 Y-20 would require the nozzle to be at X0 Y-60 so it should at least probe that point. To probe X-20 Y-20 would put the nozzle at X-20 Y-60 which is 62.4mm from bed centre, so that should be probed too, same for X20 Y-20. But all the other points on that half of of the bed will be omitted because probing them isn't possible while keeping the nozzle within the bed radius.
