Devil Deal Chance

You have a base 1% chance for a Devil Deal. No Red Heart Damage on floor means you gain 99%. No Red Heart Damage on the boss means you get another 35%. Red Heart Damage on on the boss counts as damage on the floor and the boss, so you lose 99% and 35%.

Book of Belial increases your chance by 25% when held during the boss fight (does not have to be used).
Book of Revelation increases your chance by 35% when held during the boss fight (does not have to be used).
Blowing up a beggar (even one in the arcade) increases your chance by 35% for that floor.
Having a Pentagram increases your chance by +20% each floor. Having more than one Pentagram is an additional +10% on top of the 20% from having the first Pentagram.

You can't get a Devil Deal on basement 1, unless you get teleported from a red chest or a Joker Card. Therefore, red heart damage doesn't matter for getting a Devil Deal on basement 1.

If you had a Devil Deal on the previous floor, then you multiply your overall percentage by 1/4.
If you had one 2 floors ago, then you multiply your percentage by 1/2.

If you had a Devil Deal on the previous floor and your current floor is an XL floor, then the XL floor counts as the second of the two floors and you therefore multiply your percentage by 1/2.
If you had a Devil Deal two floors ago and your current floor is an XL floor, then the XL floor counts as the second of the two floors and you have your full Devil Deal percentage.

IV Bags / Blood donation machines, Bad Trip pills, using the Razer item, using the Blood Rights item, and playing a Demon Beggar, all do not count towards red heart damage. Soul Heart damage will never reduce your Devil Deal chance.

If you get a Devil Deal but do not trade your health for a deal, then from then on you have a 50% for a Devil / Angel room. But as soon as you trade health for a Devil Deal, you cannot have an Angel Room for the rest of the run.

If you get Krampus on Basement / Cellar 1, it does not count as seeing a Devil Deal and therefore you are still guaranteed a Devil Deal on Basement / Cellar 2 if you do not take red heart damage.

If you get a Devil Deal on Basement / Cellar 1 and do not take anything, it does not count as seeing / passing up a Devil Deal and therefore you are still guaranteed a Devil Deal on Basement / Cellar 2 if you do not take red heart damage.

If you get a Devil Deal on Basement / Cellar 1 and do trade your health for a deal, you are still guaranteed a Devil Deal on Basement / Cellar 2 if you do not take red heart damage, but Krampus will now be a 40% chance (until he appears on a floor, whether you see him or not).

You should always exit the game and open it up again after every run, as the "Last Devil Deal" counter is messed up after each run. By not exiting the game and re-entering it, it's hard to guess when you will get a Devil / Angel room. And it can be an Angel room in this case as the requirement for one is that you have seen a Devil Deal in your current play session, which if you did in the past run - then you have.