To define a custom target AFR open the configuration file afr.cf in your favourite text editor, be that notepad, textedit, emacs or vi. In it you will see a number of lines similar to those below.
load=emerald,0,14.7 load=emerald,1,14.5 load=emerald,2,14.41 load=emerald,3,14.26 load=emerald,4,14.11 load=emerald,5,13.97
Each of the lines defines the AFR at the load site, after the ECU name the next entry is the load site, 0 being low load and 15 being high load. The final entry is the AFR target for that site. The default values should be sensible for most people, however if you know that a different AFR would suit you better just change the value and save the file.
If it's not an emerald that you're running look for the lines with the name of your ECU in them. For MAP based ecus, such as the megasquirt the load function has a second purpose, that of defining at what load bins you'd like to use. eg
load=ms,30,14.7 load=ms,40,14.7 load=ms,50,14.7 load=ms,60,14.7 load=ms,70,14.7 load=ms,75,14.3 load=ms,80,13.97 load=ms,85,13.60 load=ms,90,13.23 load=ms,95,12.87 load=ms,100,12.5
The above snippet defines the AFRs for bins at 30 KPa through to 100KPa, with bin spacings of 10KPa up to 70, then 5Kpa spacing above that level. On non-turbo engines the default load spacings are probably about what you're after but on boosted engines you will obviously have to space them out to cover the full range of boost that you wish to map.
There are no limits on how many load sites you can define but sites smaller than 5KPa are a bad idea. It is a good idea though if the bins you define match those in the map on your ECU.