Things You'll Need
paper, pen, monster ACs, PC bonuses to hit, 1 coffee mug or other non-sticky, non-translucent cup.
Take a piece of paper and make a table on it which looks like this. Don't make the table really big, because you're about to have to hide this information by putting a cup on top of it. For this example, assume an adventure where three heroes fight some bugbears, pirates, and a Dark Wizard. You should list as many items on the Y axis as you have types of monsters with distinctive ACs.
- r/mABCBugbears8+/13+9+/10+16+/16 <7/16>Pirates6+/11+7+/8+14+/14 <6/14>Dark Wizard15+/n16+/17+n/n <14/n>
The “r/m” in the upper left is a key reminding you that these are numbers to hit "ranged/melee." These are calculated by taking a monster's armor class and subtracting a character's bonus to hit. For example, you'll see that hero A needs to roll an 8 or higher to hit a bugbear with a ranged attack and a 13 or higher to hit it in melee.
Character C is a wizard with a familiar or other summoned creature that can also attack in combat, which has its own numbers noted next to that character's in <>.
The Dark Wizard has a very high armor class and can only be hit in some cases on a natural 20, denoted with an n (looks like it's all up to hero B!). Note that the table has a little room left under every entry. If the players find a way to lower the Dark Wizard's AC temporarily or permanently, you may want to write in the new numbers so you don't forget.
Now cut out this table at about the size of a coaster, pour yourself a drink, and put your cup on top of it. Your cup should be heavy enough that it won't fall over and opaque enough that nobody can see your notes beneath it. Practice picking it up a time or two to make sure the paper won't stick.
When you're playing later and players start making attack rolls against your bugbears, pirates, or Dark Wizard, you'll be able to check if they hit with discretion. Try to internalize the numbers so you don't end up drinking every time players roll the dice. If you find yourself picking up your cup too often, try hiding this sheet in your palm, sleeve, or lap.
Even if you have a DM screen, this will help speed up the game because you'll know where your reference information is. Rather than having to rifle through a pile of notes, open another book, or search for a section in a digital document, the numbers are right there, ready to keep your session flowing.