Sounds your calculations are very close to this spreadsheet I found on twitter, which has the Cs approximately $650K under the cap:
View attachment 39789
His calculations also take into account the following bonuses:
1. $1.3M Jaylen Brown All-Star
2. $0.3M Jaylen Brown Conference Finals
3. $0.4M Jaylen Brown 65 GP pro-rated
4. $0.5M Marcus Smart BF%
5. $0.15M Evan Fournier 60 GP pro-rated + 75% GP Playoffs
6. $0.15M Evan Fournier (5. criteria) + Conference Semi-Finals
7. $0.15M Evan Fournier (5. criteria) + Conference Finals
but do not take into account these bonuses:
1. $0.4M Jaylen Brown 49 Ws pro-rated & Conference Semi-Finals
2. $0.2M Jaylen Brown NBA Finals OR All-NBA
3. $0.15M Evan Fournier (5. criteria) + NBA Finals
4. $0.5M Evan Fournier (5. criteria) + Championship
View: https://twitter.com/arampalamoudian/status/1375560248333770753