Hm, as I understand it, if one party won all 299 direct mandates at 0% 2nd vote share, the other parties' seats would have to be increased until 299 == 0%. Depending on how you round, that would be at least 30,000 seats (there's some tolerance allowed that I did not consider here but that's the ballpark for that unlikely scenario).

The linked site lists a (more realistic) scenario already, where the number of seats comes out to more than 900. How did you end up at your 897 seat upper bound?