Hehehe. I didn't want to quote specific numbers because they are highly variable and depend on light, CO2, sensitivity of fish, how much you are currently dosing, etc. But anyway, before I address that, let's get some background first so I don't sound like a lunatic. If you want to skip straight to the answer then read paragraph #5. Hehehe.
If you want a plant to put out more leaves, then you add NITROGEN. That's true for most (maybe all?) plants. Our goal then is to induce the plants to take up NO3. In our system, three things come to mind: light, CO2, and PO4.
(1) Light is usually fixed in our aquariums. It ultimately determines how fast NO3 is consumed.
Hemianthus callitrichoides requires a minimum of moderate light. More light of course will help accelerate its growth...up to a point.
(2) CO2 is usually also fixed in our aquariums (20-30 ppm). Given the difficulty of obtaining
H. callitrichoides, if you have the plant, then it's probable that you are not a newbie; and unless you live under a rock, your CO2 is already near 30 ppm.

Adding more won't help the plants grow any faster, according to Thomas Barr's practical experience.
(3) That leaves us with PO4, a factor that we can vary quite easily. As I understand it, PO4 is necessary for the physical removal of NO3 from the water column. So with not enough PO4, the plants can't take up NO3 to grow, even if there's a bunch of it floating around. Therefore, add more PO4. And here we are, how much to add? Research has shown that under MAXIMUM light (full sun), the most NO3 that the plants can remove PER DAY is 5 ppm. So add 5 ppm NO3 along with ~0.5 ppm PO4 daily: no need to be uber accurate here.
So, do you know how much NO3 your tank is using daily right now? If it's near 5 ppm, then you are already at the maximum growth rate. If it's less, then you need to increase light, CO2, and/or PO4. As stated above, most of us have light and CO2 where we wanted so PO4 is your only choice. But UNDERSTAND that light and CO2 ultimately determine how fast your plants grow.
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DISCLAIMER: There are repercussions that can result from adding so much macros.
(1) Increasing PO4 will only drive the physical uptake of NO3 into the plant tissue. You must provide adequate K and traces as well for the plants to convert this NO3 into vegetative growth.
(2) Etc.
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I hope that was thorough enough. Please correct any mistake and omission.