Following a message board post on alla’s paladin forums, I quickly put together a spreadsheet calculating the proper average heal values including spell bonus and downranking:
Since I tend to fail at maths, you are invited to review the spread sheet here. The downranking formulae have been taken from http://www.wowwiki.com/Downranking. If you wish to simply check your normal heal average by level or spell damage, simply modify the values in the two blue fields to play with the numbers.
Of course, in-game, you will get a lot more information including taking crits into account by using add-ons such as DrDamage. Still, I hope this isn’t too far off with the maths and comes in handy when you don’t have access to your DrD-modified tooltips.
Corrections? Let me know, I beg you.