With everyone so mobile atm I can only imagine every team fight ending up being a clown rodeo trying to get meaningful heals off while your teammates bounce off the walls like a bunch of first graders on crack. I think the Shield aura rewards players enough for being close to Sona. I think the heal just needs a flat ratio + base nerf because of all the Sheild + heal amp with ardent and mikeals.
Actually, now that I think about it, why not just give the heal a flat nerf, base heal and ap ratio heal, but buff the shields ap ratio so that it encourages people to actually stay in the aura of the "aura support." I understand that you want people to benefit from actually being in sonas aura, but I'm not sure that nerfing the projectiles efficiency at ranges would be the best option. Maybe Nerfing the W projectile range as well as the ratio and base heal would do the trick. Maybe a system where the Heal will not connect if the target leaves the max range of it?
Using your Idea a little bit, why not just make the Heal do less if they aren't in Sona's aura at all? Having the heal do more when closer is not inherently a bad Idea, but making it scale off of range like a hydra in my opinion does.
I think the values that you are using aren't a very good match with the 125 increments. If someone is Right on the edge of Sona's aura, then the heal would be doing 96 + .21 AP. If they move 25 units closer, then they will be healed for 104 + .2275 AP. Those 2 values would be the usual if they are inside your aura. Its not very common that a teammate would stand directly inside of you for a heal lol. Also, that's at max lvl. Since Sona maxes Q first, the Heal would be practically nonexistant during lane phase. 48+ .21 ap / 52 + .2275 ap until it forces you to put another point into it. Sona needs to max Q first because her heal + shield is pitiful compared to other supports who specialize with those 2. Her Q is her overall strongest ability and needs to be maxed first if you want to win lane. Also, people moving back and forth through the ranges of your W might screw up the healing that It will do. A system like this would destroy her consistency when it comes to heals.
Maybe I should rephrase what I said in my previous post. I don't think that making a system like that would work with the heal projectile, however I think that there are better Ideas to promote Sona's aura playstyle.
I just think that her Q and W ratios need to be nerfed, The W projectile needs to be hit pretty hard with a base and AP ratio nerf, but the shield might need a little bit of a buff to compensate for the W nerf. Her biggest problem is her scaling. Her ult allows for her to become a late game hypersupport carry. That wouldn't be such a problem if her values were lower.