Latest day of Easter is April 25.
There are 243 days between Easter and Christmas
Every 400 years the difference between Gregorian and Julian calendar increases by 3.
Year 300 AD is the reference point for a difference of 1 day between calendars

Example
Year Difference
200 0
300 1
400 1 (leap)
500 2
600 3
700 4
800 4 (leap)
900 5

Given x the target difference between Gregorian and Julian
(\floor((x - 1) / 3) * 4 + 3 + ((x - 1) % 3 + 1)) * 100

```python
bucket = (x - 1) / 3
projectedBucket = bucket * 4 + 3
positionInBucket = (x - 1) % 3 + 1
finalPosition = projectedBucket + positionInBucket
actualYear = finalPosition * 100
```

let x = 243 => actualYear = 32600