I can't provide any input re. watering (I grow in hydro) but, to your question, about 30% — that's very dry for cannabis. Your plant is in the vegetative stage and it will do best in 50% 60% humidity.
Humidity is only part of the question, though, just like it is for humans. You're right about low humidity increasing transpiration but there's also temperature to take into account. In the same way that humans feel dry air or wet air, plants do best in when temperature
and humidity are in a certain range. That's the idea behind "vapor pressure deficit" ("VPD")
Plants, again like humans, are pretty much a bag of water. When it's really hot and dry, transpiration/evaporation goes up. When it's cold and damp, there's very little transpiration/evaporation. VPD is the way to figure out if a combination of temperature and humidity are "just right" for a plant.
A good source of info on VPD is
this site. Poke around in the "Learn" section and you'll get smart on how you can have different combinations of temperature and RH are in a range of optimal values. Per the citations on that site, optimal VPD can increase growth by 20%±.
Your plant is also dealing with not enough light. Classic symptoms of not enough light are that the plant has limited foliage, is tall, and has a large space between the nodes ("large internodal space").
When cannabis doesn't get a lot of light, you'll end up with branches that have small, airy buds, scattered along the stem instead of large, dense colas that form when lots of light allows the buds to grow large enough that they actually grow together to form one large bud.
Fortunately, the light issue is easy to fix. Unless you have a reason not to, you might want to follow the manufacturer's
guidelines. What's the make and model of your light?