So after doing the campaign, there are a number of quests that reward exotic weapons (MIDA Multi-Tool, Sturm, Rat King). I've noticed that these rewards scale to your power level to some extent after I got a 280 Sturm on one character and a 294 Sturm on another. Is there a hard and fast rule to see how powerful your quest reward is going to be or is it more RNG-based?
Your Total Base Power (TBP) is based upon all of your highest Base Power Level equipment, this number will determine the Power Level of items dropped. Legendary (+5) Mods do not count toward your TBP. This will look for all of your highest Base Power Level gear throughout your vault, inventory, and equipped items, feel free to use any equipment you'd like! Furthermore, each slot may be filled with an exotic to count toward your TBP.
Your TBP will be the average sum of all the highest Base Power Level gear. (This number can also be found by logging into Destiny Item Manager.)
Subtract 5 from your TBP to get your Drop Power Level. (Ex: 280 Total Base Power = 275 Drop Power Level)
Drop Power Level Drop / Reward Type +0 Rare Engrams, Legendary Engrams / Rewards +3 Crucible Clan Reward +5 Exotic Engrams +8 Nightfall Clan Reward +10 Personal XP Clan Reward +13 Powerful Gear Milestone Reward +18 Exotic Quest Reward +?? Raid Reward
If your Drop Power Level is 275 and you complete a Powerful Gear Milestone you will receive a drop with a Base Power of 288.
These numbers may differ when you reach 285-300 TBP.