Method: 1 unit of archers shooting at 1 unit of infantry until the infantry make contact, then pause at note the number of remaining infantry out of 70 starting, test run on Grassy Plain in clear weather and midday. I adjusted the number of upgrades and armor stats to try to see if any produced the same average number of casualties indicating equivalence.
Results:
Test 1 - inf base armor 5 - armour_ug_level values: 2, 3, 4, 5 - 0 upgrades given: 44 48 53 57 avg. 50
Test 2 - inf base armor 5 - armour_ug_level values: 2, 3, 4, 5 - 1 upgrades given: 54 57 60 60 avg. 58
Test 3 - inf base armor 5 - armour_ug_level values: 2, 3, 4, 5 - 3 upgrades given: 63 62 64 67 avg. 64
Test 4 - inf base armor 5 - armour_ug_level values: 2, 3, 4, 10 - 3 upgrades given: 64 66 66 62 avg. 64
Test 5 - inf base armor 5 - armor_ug_level values: 2, 3, 4, 1 - 3 upgrades given: 64 65 66 66 avg. 65
Test 6 - inf base armor 8 - armour_ug_level values: 2, 3, 4, 5 - 0 upgrades given: 59 59 61 61 avg. 60
Test 7 - inf base armor 11 - armour_ug_level values: 2, 3, 4, 5 - 0 upgrades given: 66 66 60 63 avg. 64