I'm having difficulty with one of my Mastermind's Dark Miasma powers. Twilight Grasp, the debuff and heal. I have it slotted with some accuracy to help it hit more often, but the healing of my minions seems totally random. When it lands it always heals my character, but it only seems to heal my minions1/4th of the time. Do I need to be standing within a particular distance or something to make it heal more often? I don't totally understand the mechanics of the power.

Sounds like you've almost got it figured out.

The power siphons energy off the target, sends it back to you, and heals all allies within about 15 feet of you. Since you are always within 15 feet of you, it will of course always heal you. Your pets (or teammates) need to be nearby when you activate the power to get the benefit.

Since you have (or will get) Shadowfall, which also requires a tight radius, I'd suggest you try to get the hang of positioning yourself and your pets in the optimal area. I have a dark miasma defender and she bounces all over the place trying to get people healed when they aren't grouped well. With pets, you'll have to figure out the right balance of keeping them close to heal, or sending them to go melee fight (but possibly dying).

I play a Ninja / DM mastermind, well two of them, but anyways. The trick to keeping your pets healed can be done one of two ways. If they all need healing, simply use the Guard command, don't remember what the macro is exactly, but basically, it tells the pets to stop fighting and come to you. At that point, you can do a mass heal.

If its one or two pets, you can simply target the pet thats injured, move near it and then heal. For the most part, I don't even worry about my lesser pets dying. I usually try to keep my Lt's and Boss pets alive and if all my lesser pets die before the end of the battle, I simply resummon them.

If you target your pet, it will automatically target the mob that the pet is targeting. This is a nice boon for melee pets as it will target the one the pet is attacking, thus healing any pet, team member and minion around the enemy.

The auto assist works both ways. Target an enemy, cast a heal on the enemy and it will cast on the target of the enemy, thus healing the team mate, minion or pet.