HTP vs MoN

It's not a matter of level, it's a matter of your current stats.

What % of your current w.att is your MoN? For example, if your overall w.att (including weapon, cape, glove, rings, belt, shoes, shoulders, mask, Blessing of the Fairy, and any potions or buffs you usually train with) is 200, and your MoN is the average 10 w.att, then your MoN is 5% of your w.att and therefore of your damage.

How many %DEX do you already have on your gear?
In the extreme case that you have 0% DEX, a 3% dex HTP might already match your MoN (because of the extra 17 dex and str it has), and a 6% definitely will. On the other hand, if you already have 100% DEX, every additional % counts for less, so to match those 5% damage you'd need at least 9% DEX, possibly 12.

Or in simpler terms

1. Get Total ATK value
2. Estimate New Items' ATK value
3. Calculate:
(Old Items' ATK - New Items' ATK)/Total ATK = % needed to cover.

In this case, "New ATK" would be 0 and "Old ATK" would be 10.