(note: all dmg taken below excludes dmg taken by my MC, who has higher DEF)

dmg taken with SR Philosophia as sub ally:
1401
1468
1406
1431
1463
1388
1393
1445
1383
1423
Average: 1420.1

dmg taken without her:
1497
1452
1442
1521
1506
1467
1527
1523
1538
1490
Average: 1496.3

1 - 1420.1 / 1496.3 = 0.0509256165
So SR Philosophia reduced dmg taken by about 5%.