My experience with silver sheet is that you want to get the lowest nickle content possible. The silver I used to use had high enough nickle to cause it to be very hard - the cutting with a jewelers' saw was very slow going.
Even pure silver requires pretty high pressures to be ductile. For any diameter of coin, the press requirement goes up with the square, naturally, but for hardness of material the press requirement goes up exponentially as well.
Now to answer your question. If you are just stamping the letters, then the pressure required is the Volume of the indent, with a factor for the square area of that image, times a constant. So if you have a logo like the one pictured, then it gets very high quickly.
Have you considered a striking die to be used just after is sets up, say at 800 F? This does not require a press or even a fancy alloy, even cold rolled will do...