The example is clearly wrong. The stated rule seems to gives very high breakage numbers, but that might be deliberate. I get 105 * 1.5 = 158.
For comparison, the RMU rule similarly adds a modifier for quality and materials, but rather than giving a fixed multiplier for all magic items, adds the item's level (which is equal to the highest level spell used to create it). Using a fixed x1.5 multiplier means a +5 magic sword could have the same strength as a +30 magic sword (assuming they are made from the same material, which they probably aren't), but also means that items with a high strength get more benefit from enchantment than low strength items. E.g. a broadsword with a strength of 80 gets a boost of +40, but a club with a strength of 50 only gets a boost of +25 from the same enchantment.
Downside of adding the item's level, of course, is that you have to figure out the item's level.
