Honestly at level 40 the math is actually super easy to show that regrowth is going to be better even if you don’t downrank
Assuming 200 +healing for easy math and using the highest ranks available to you:
Regrowth (Rank 5): Will heal on average 514 up-front, non crit, plus 527 over 21 seconds; a total of 1041. Assume an 8% base crit chance based on roughly 200 total int plus 50% with talents. Mana Cost: 420
Healing Touch (Rank 7): Will heal an average of 1212 up-front. Mana Cost: 405 (365 with 5/5 IHT Talent)
Assuming you do not spec Living Seed, with a crit chance of 8% at about 200 Int, you average 1309 healing per 100 casts of HT.
Assuming you do run Living Seed, with an 8% base crit chance plus an additional 50% to Regrowth, the average Regrowth direct heal will heal for 778, with an approximate proc value of an extra 233 healing from the Seed proc, plus the additional 527 HoT for a total of 1538 healing.
In conclusion, running Living Seed and 5/5 in Improved Regrowth, you heal approx 3.67 health per mana with Regrowth
HT, with 5/5 Improved healing Touch, will heal about 3.59 health per mana
The difference is slight, but with Living Seed + Regrowth you additionally gain the benefit of being able to use Nourish consistently without setup.
This math changes based on rank, but as Regrowth SP mod is the same per rank (unlike HT), lower ranks of the spell actually become more mana efficient as you gain Spell Power, especially if used only to have something on a target to buff future Nourish casts.
EDIT: All of this information is fact-checkable by simply pulling up the spells yourself on wowhead/vanilla and plugging a 200 SP coefficient into the math on a calculation, then add (0.5 * x) where X is your crit chance to the up-front portion of the heal to see your own average cast value