High light always means a lot of work, compared to low light.
What I posted is, "You can see, from that chart, that at 33" you are getting about 70 micromols of PAR, which is more than enough light to grow hairgrass. If you were to use just an adjacent pair of bulbs you would get about half of that, or 35 micromols, which might be too little light. But, lower the light to about 22-24 inches from the substrate and you should have about 45-50 micromols of PAR at the substrate from the two bulbs. That is plenty of light for hairgrass, and based on the charts in the manual, the light would be adequately uniform over the whole substrate." This means, with all 4 bulbs on, and the light 33 inches from the substrate, you get about 70 micromols of PAR, which is plenty of light for hairgrass. If you want to use only two bulbs, you can lower the light to 23-24 inches from the substrate and get 45-50 micromols of PAR, which is still very likely to be enough light for hairgrass. If you don't mind using all 4 bulbs for the whole photoperiod, just leave it at 33 inches and run all 4 bulbs for the whole photoperiod for 70 micromols of PAR.
I appreciate you explaining it this way - will gradually move towards dropping the light and reducing the burst now that I've upped the flow and CO2.