Question about open reservoir and evaporation

So I'm thinking about my irrigation system and feeding .

I have roughly 50l containers which I've attached steel racks or old fridge trays will holes drilled and I'm going to have raised grow bags ontop .

From about week 4 I'm going to fill the containers with about 8litres each with mixed nutes and use blumats to feed for the next 4weeks .

I'm using this set up as feed and run off aswell but there probably won't be any run off with blumat except thats what I'm hoping .

So basically I will have an open reservoir as that's what I have at the minute and it all kind of works ...

The only concern is evaporation.

My tent is at max 26 degrees under light but have yet to have my ligh lt running high but 26 is were I'm aiming .

So I'm wondering how much extra water should I account for. Running an 18/6 for 4 weeks .

The first 4 weeks I will just take out and feed .

(Growing autos by the way so flushing by about week 9/10/11 or just cutting all nutes and refilling reservoir with ph water .)

Evaporation isn’t the only problem. Green algae is real common too. So your plants are in the totes on racks above the solution, right? Unless you could use something like panda plastic and tape it to the sides of the tote under the plant. It’d be a bit of cutting and taping, but it could work. Fluid changes would get tougher though.
If you’ve got no problems yet, then it might be ok the way you are.
As far as extra water to account for just mark a spot on the side of the tote and top up with water to that point either daily or every few days depending on how much they drink. I run my hydro reservoir levels like that.