Cross posting this from somewhere else. Helps to explain the forge a little bit for those unfamiliar. Cliff notes are unless you are high vip and willing to spend gold to forge, DO NOT upgrade forge past 103.
First, on a general basis, forging with more jewels would give you a better chance to increase the forged attribute value. The better mode, the better chance. Note here, I use the word "chance". There are exceptions that I explained below: one is when your forge is not higher than Lv103, the other is how to use Gold Mode to boost your equips.
1. Forge Lv103 is a critical point. There is no need to upgrade it before the extra attribute value reach 3*growth value. (Growth value refers to the value increase after upgrade. For example, growth value of a purple weapon is 28. So, there is no need to upgrade Forge above Lv103 before you get 28*3 extra ATK on your purple weapon.) This will make your forging more efficient.
2. When your forge is not above Lv 103, the effects of Platinum and Supreme modes are the same. This is only true when your forge is no higher than Lv103. So, if you want a better result, 10 jewels with Platinum mode are your best choice, when your forge is no higher than Lv103.
3. People may wonder how are 50 jewels better than 10 jewels. Here is an example. You are forging your purple weapon. Your forge is Lv110. You have added 60 ATK to the weapon through forging. Now in common mode, forging with 10 jewels will get you a number within [0,67], forging with 30 jewels will get you a number within [4,67], while forging with 50 jewels will get you a number within [18,67]. This is just an example. Things will be different when the forged value is big enough.
4. If you accidentally upgraded the forge above Lv103, Superme Mode with 30 jewels or 50 jewels will guarantee the attribute increase on the equip. (Of course, 50 jewels would have a better effect.) This rule will always be correct if the forged value is no higher than (Forge Lv-102)*growth value.
I could not disclose all the formulas. Hope this could help you use this function better.