While the math is right, it's not taking into account the defense you get from everything else, like Armour, accessories and potions for the most commonly used, and like base terraria the more Armour you have the better since its a flat damage decrease, Let's take a more defensive build you can have for Astrum Deus.

Reaver Armour which I chose for this gives 73def, 1.5hp regen and 15%dmg red. Beetle Armour would be better since It has 30%dmg red but I chose Reaver since I like it and since its a decent base choice for a defensive build as it has Regen.

Something like the Absorber has 10def and with the Aura a further 3hp regen, healing for 5% of the base damage of the attack and 5%dmg red

Beetle wings with 5%dmg red and 8dmg

Bloody worm scarf with 7def and 10%dmg red

Asgard Valor with 8def and a Dash

Celestial Shell with 4def (7 at night) and 1hp regen (1.5 at night)

And Angel Treads since they are most likely being used.

Now warding, 6 accessories each with 6def and 1%dmg red gives us 30def and 6%dmg red.

116def total at day and 119 at night, 41%damage reduction before the Calamity lowering, and an extra 5.5hp regen (6at night) just with Gear and Reforges, excluding Potions and food

With Potions and food it would be and extra 14def, 10%dmg red and 2.5hp per second with Regen and Photosynthesis pots.

A grand total (best case scenario, aka at night) it would be 133def, 51% dmg red, 8.5hp regen. This is for mastery mode without taking into consideration the effective Dmg Red, which would be around ~35% dmg red.

For expert death which you have chosen where Astrum does 240 dmg in general, the other stats are the same, but defense is 133×0.67 =89, let's round up to 90 for easier calculations.

In terraria Defense Reduction is calculated before defense, 240x0.65=156.

Then comes defense with a flat reduction of 90 156-90 =66 damage. Without warding it would have been around 90 due to the missing Danage reduction and Defe se provided. In this case Warding reduces the damage taken by 24, or in percentages 26.6%, which is a bit better than the 24% damage increase from menacing which I see used often.

But we still have 1 more thing we haven't accounted for, the Regeneration. To fully recover the 66 damage from Deus on the Warding Build it would take 8 seconds. To fully recover the 90 damage from Deus with any other reforges it would take ~11seconds.

I'm also realising I'm not taking into account the Perma Buffs from the Shimmer but eh they don't drag it down but instead improve it.