About the stamina/xp ratio, the more stamina a campaign mission takes the better the ratio will be. However at this stage girl xp is probably more important.

->Open Spoiler<-

Stamina XP Ratio

21 254 12.10
22 268 12.18
23 283 12.30
24 298 12.42
25 313 12.52
26 328 12.62
27 343 12.70
28 358 12.79
29 374 12.90
30 390 13.00
31 406 13.10
32 422 13.19
33 439 13.30
34 456 13.41
35 473 13.51