No way id water those plants until 20% runoff in that medium. How much have you been watering/frequency/amount? At the size they are Id not give more than a 1/2-3/4 gallon water. 2-2.5 gallons would need to be a huge flowering plant. Under 600w LED. 1000w HID.
Generally I grow trees, in 20g Promix BX 1000w HID. In a 20g container, I NEVER give more than 4g, and they use it in 3.5 days at 78f - 84f. Fans blowing 24/7 even on 12/12. I veg 24/7 x 1000w HID
How much Nitrogen are you giving??? Mg deficiency usually happens all over the plant. N deficiency starts at the bottom, and goes up. Generally.
Also if you are using the 7-4-10, I feel it is lacking in Phosphorus, and K. Weed generally likes a 1-1-2 ratio,... 7-7-14 would be better. Weed is really similar to Tomatoes. Weed just likes a bit more N, and K Not much.
I use a 19.5 - 20 - 39 ratio, and adjust PPM. I feed at every watering. I never hardly water to runoff.
I use a cheap Lowes ect Moisture Meter, and have for 45 years. I poke holes in the lower sides of containers, as well as take readings from the top of the container. I NEVER let the moisture level get below the LOW MOIST LINE, and dont let them stay wet for longer than 3.5 days.