The original statement was;
Ischaldirh wrote
As I recall however enchantments boost damage by a flat amount, rather than a percentage
And the formula is clearly percentage based. Also, precisely because it's percentage based, a stronger weapon will not follow the same pattern as the leather whip used in the example, one of the weakest weapons. To illustrate this point, I tried enchanting Turox, one of the most damaging weapons even without the explosions. At +2 enchantment you've gained an average of 0.5 damage, not 1. To balance this out you need to get it up to +4, at which stage you gain an average of 1.5 damage. On lower levels it's not much trouble but at higher levels it'll be hard to get to that balancing point because it's a struggle just to get that +1 enchantment.