The only time "Maximum Damage" occurs is during a Citadel hit. "Pen" damage commonly comes out as 33% of rated value, and 16.5% when striking a Saturated Section (the HP of the section is depleted, but you are still dealing damage to the "Base Hull")

So, your Sherman with 2,800 Shells is dealing 33% on a Pen

2,800 x .33 = 924 damage per Pen

After the HP of the middle section of the Kleber is depleted (French DDs do allow for mid-section saturation), the damage would be cut in half.

2,800 x 16.5% = 462 damage per Pen

Assuming all 25 shells were Pens, you would have a damage of 23,100 HP.

924 x 25 = 23,100 HP

If half (we'll say 13) were full pens, and half (12) were saturated pens, you'd have

(13 x 924) + (12 x 462) = 12,012 + 5,544 = 17,556 HP

Further, you could deplete the middle section AND the "Base Hull", which would make Fully Saturated sections receive only 10% of rated damage. That would be 280 damage per hit.

You can learn all about HP stuff in this video: