I suggest you to look up for a VPD chart. Vapor pressure deficit is a value with which you measure the right transpiration of the plant. With that chart you'll know what humidity you should have with a given temp and vice versa to give the plant the best transpiration possible in the stage you are in.
Best way to raise the humidity is a humidifier of course, if you can't afford it, place a bowl of water in the tent and/or place a dome on your seedling so the moisture of the soil remain in the dome.