How to gain most XP during double XP or rapid respawn events
Start from 42h of stamina at the server save, and finish at 14h at the server save of the last day of the XP event. Optionally you can buy up to 2 XP boosts per day (+1 boost from the day before, as mentioned in the text above). Buying 3, 4, or 5 boosts during 1 day is less efficient due to high price than buying single boosts (1 per day) during regular days (without double XP or rapid respawn).
Efficiency of XP boosts in relation to price
It is more efficient to buy 1 boost during a regular day, than to buy 3rd, 4th, or 5th boost during double XP event (or rapid respawn event).
Buying XP boosts during a regular day:
13,333k/h base XP (20k/h with green stamina, without XP boost)
1st boost: +6,6k XP for 30tc (4,5tc per 1k) [30k/h with green stamina]
2nd boost: +6,6k XP for 45tc (6,8tc per 1k)
3rd boost: +6,6k XP for 90tc (13,6tc per 1k)
4th boost: +6,6k XP for 180tc (27,2tc per 1k)
5th boost: +6,6k XP for 360tc (54,5tc per 1k)
Buying XP boosts during a double XP event:
26,666k/h base XP (40k/h with green stamina, without XP boost)
1st boost: +13,3k XP for 30tc (2,2tc per 1k) [60k/h with green stamina]
2nd boost: +13,3k XP for 45tc (3,3tc per 1k)
3rd boost: +13,3k XP for 90tc (6,7tc per 1k)
4th boost: +13,3k XP for 180tc (13,5tc per 1k)
5th boost: +13,3k XP for 360tc (27,0tc per 1k)
Source: https://www.reddit.com/r/rookgaard/wiki/index/exprate/