Which, in 5e, I can determine up to CR13 or so using 5e's own rules. But what is a 5e CR 17 monster equivalent to? I dunno, because 5e doesn't even tell me how tough that is in 5e.
There are four CR17 monsters in the Monster Manual - Adult Gold Dragons, Adult Red Dragons, Androsphinx, and Dragon Turtles. A CR17 monster is meant to be equivalent in strength to a party of four 17th-level player characters.
Hit points for all four are 256/256/199/341
Armor clases are 19/19/17/20
Assuming 1 out of every 3 attacks misses, damage average per turn are (roughly) 34/34/34/42 dmg (the hit modifiers are +14/+14/+12/+13)
So a CR17 5e monster has anywhere from 199 to 341hp, from AC17 to AC20, and does between 34 to 42 damage per round on average.
For reference, a control Bugbear of CR1 has 27hp, 16AC, and does roughly 11 damage on an average hit (at +4 to-hit).
So you can compare the average monster stats to the average player stats (by level), find an equivalent value in 4e terms, and you can adjust accordingly.
FYI, the table on page 274 of the 5e DMG might be extremely handy for you right now - average stats for monsters up to CR30. Same with the "Experience Points by Challenge Rating" table on the next page that also goes up to CR30.