Filling tank for first time

[daplatapus]

I guess there are a couple ways to do it. I'd fill my display tank until it starts to go over the overflows. Then fill your sump to the safest point it allows.
This stage simulates a power outage in reality. No water flowing. Fire up your return pump. If you need to add more water, something in the design is wrong because if the power ever goes out you've just added more than the sump can safely allow and will probably overflow. So it's important to fill your sump the first time to the maximum safe limit then fire it up and add no more than that.

That being said, you'll want to mark the return chamber of your sump at the level it stays at once the system is running for 15 minutes or so. This is where you'll notice any evaporation. If your tank is marked all you need to do is top it off to that mark with fresh RO/DI water as you notice it drop due to it evaporating.
Hope that helps.