This is currently being revised as the true number has been determined to be closer to 36.25, both it and 36.4 come to the same damage result for the example move used below.
Through some testing with moves that deal set damage, I have found the standard amount of damage reduction all players have from damage dealt by other players to be 15%. This means a move like vampire's Freeze that would normally do 25 damage would instead do 21.25, which is displayed as just 21.2. Ultimate Life Form and its True counterpart both have a damage reduction against players that is meant to be 40%, which would cause the damage dealt by Freeze to instead be 15 when not considering the additional 15% reduction, however when tested the number appears as 15.9 which is not what would be expected. Through a few calculations I discovered that the damage players reduce from each other normally is added on wrong. Normally the way it would work if done correctly would be the damage would be reduced by one and then another, with the second amount in this case being the one added later in the game's life, the base reduction. For an example, lets say you were hit by something that should do 100 damage, but the 40% reduction comes in and lowers that amount to 60, the additional 15% is calculated separately with 60-15% being 51. The result shows that the two separate percentages can also be put together as one being reduced by the other, then the reduced one added to the non-reduced amount in this case being 15% reduced by 40% to become 9%, which should then add onto the 40% and become 49% total. However what we observe is that the non-reduced amount is then reduced as well by the second amount rather than being added together, causing 40% to become 36.4%. Due to this ULF and TULF are both less effective at reducing damage than they should be. I do consider this a pretty serious bug, however I do not expect it to ever be fixed any time soon.
TLDR: The math is actually pretty simple, but for more context read the whole thing. Just do 15-40%, then 40 minus the previous answer, which will be 9%. It comes out as 36.4%, which is the exact amount of damage reduction ULF and TULF, which should not be the case, rather it should be 49% combined, but the devs made an error somewhere.