Versions:

• Construct's Armory: 1.12.2-1.2.5.10
• Tinkers' Construct: 1.12.2-2.13.0.183

Observed Behavior:

In the code https://github.com/illusivesoulworks/constructsarmory/blob/master/src/main/java/c4/conarm/common/armor/traits/TraitMagnetic.java#L43 there is a formula: double range = 1.8D + (level - 1) * 0.3F; so Magnetic II gives 2.1 blocks of vacuuming.
In the game after testing with 2 pieces with Magnetic I and with 1 piece with Magnetic II can confirm that magnet range was near 2.

Expected Behavior:

Wiki says: "Magnetic: Pulls in dropped items within a range of {1.8 + {number of armor pieces – 1} * 3} blocks. Recently dropped items are not affected until the pickup delay expires."
So expected range with 2 pieces with Magnetic I and with 1 piece with Magnetic II was 4.8 blocks.

I tested with different total number of ppieces of armor and level of modifier and can confirm that actual formula uses 0.3, not 3.