>> I got a lot of condense (water drops on the plastic foil
Calin, here is my opinion, but I'm not an expert. I think the humidity is right when there is just a LITTLE condensation. Like light fogging or the plastic. Not big droplets, Not heavy mist. Just a little fog.
I think we need to keep the soil surface from drying out. We don't need to keep it soggy or dripping. We don't need 100% humidity in the air above the soil. (Once seedlings emerge, high humidity in the air is a very bad thing - it encourages damping off disease.)
Big drops of water condensing on plastic film or a humidity dome suggests that maybe the soil surface is more damp than it needs to be. When I get that, I try to get the drops to run off so they do not fall back onto the try, and I use a turkey baster ("syringe") to suck any water out of the tray under to pots. Also, I prop up one or two corners of the clear dome (or pull back the plastic film from one end) to let some of the excess humidity diffuse out of the air and away from the tray.
No condensation at all suggests that the soil, or at least the soil SURFACE is quite dry and might be too dry for the seeds to sprout quickly. That's when I spray a little water on the surface, or add water to the pad on the tray under the pots.
I think that the best sign is when the plastic film or humidity dome just shows a little mist or fogging. Maybe just part of it will fog up, or most of it fog lightly. I think that's ideal.
Of course, there are things that can confuse this issue. If warm light falls on the plastic cover, that might prevent condensation no matter how moist the soil is. If the clear plastic is warm, water will not condense.
When you have a heat mat under the tray, and cold drafts on the clear plastic, I think that you get a lot more condensation, even if the soil is on the dry side. So with heat mats, I want to have somewhat heavier fogging, and maybe a light mist. But not big droplets.
Here's another confusing factor: if you have mist on the plastic, just mist and no visible droplets, if you tap-tap-tap the plastic, you can probably make the mist collect into droplets big enough to see. That kind iof "doesn't count".
