I am working on a homebrew monster and reading the Dungeon Master's Guide p. 278 states I can
Use the Table. You can start with the monster's expected challenge rating and use the Monster Statistics by Challenge Rating table to determine an
appropriate save DC for any effect that requires a target to make a saving throw.
Calculate the DCs. Alternatively, you can calculate a monster's save DCs as follows: 8 + the monster's proficiency bonus + the monster's relevant ability
modifier. You choose the ability that best applies. For example, if the effect is a poison, the relevant ability is probably the monster's Constitution. If
the effect is similar to that of a spell, the relevant ability might be the monster's Intelligence, Wisdom, or Charisma.
Don't worry if the save DCs aren't matching up with
the expected challenge rating for the monster. Other
factors can affect a monster's challenge rating, as
shown in later steps, and you can always adjust the sa –
DCs later on.
So let's say I have a monster CR 17 with a 30 con score, and I believe the con score should be used for the ability. Calculating the save the DC would give me 8+6(proficiency)+10(con)=DC24. Alternatively I can simply use the table which gives a DC of 19.
Do I just get to pick whichever one best suits my monster??