Runeforging Brass Dome - Where does the value come from?

So I wanted to make a runic ward build with brass dome, but I cannot for the life of me figure out where the value of the ward after forging comes from.

I tried using high armor base and it got a low value.

I put another 4000+ armor Brass Dome in and it previews 890 ward.

I divined it to see if the value was rerollable and the preview didn't change with a new armor value or item rerolling.

Is it just a built in value to the individual item that can't be rerolled 750-1000? I don't understand whats going on here. Is the only way to find a good base to buy them and preview?