If leveling on Marble, make power and strength scales. If leveling on Mithril (sounds like you have enough to get to lvl 100 easily) make Dexterity and Focus scales.
Use whichever formula's item you are just past optimal to make. Don't use something you are not optimal on, even by just 1 bar.
Optimal vs not example. You make a scale that at optimal requires 12 bars, you are not yet optimal so it uses 13 bars. When you are optimal, it takes 12 bars to craft, and you will get 6 back when deconning. So 6 bars used. When not optimal, it takes 13 bars to craft, you get only 5 bars back, so 8 bars used. That's 25% more resources. Making an optimal item might give a bit less exp per item, but it will never exceed the 25% threshold to be worth it.
For Dexterity scales, these are the skill levels needed for optimal:
1020 Dexterity Chest/Head/Foreleg scales
1120 Dexterity Back/Tail/Hindleg/Wing scales
For Focus scales, these are the skill levels needed for optimal
1050 Focus Chest/Head/Foreleg scales
1150 Focus Back/Tail/Hindleg/Wing scales
They are all the same in the bars needed, so 1 formula from each grouping is all you need. So dex chest, focus chest, dex wing, focus wing for example will get you by if working with just Mithril. Switch items as soon as your current Scalecraft skill hits each level. Using biped buffs add skill:
Gift strength V, gift power V, enhance strength V add a total of 15 Scalecraft skill.
Determination, Promote Vitality, Promote Intellect self buffs (Promote Intellect is newer and you likely do not have it, but its easy to find someone to make you one) add 22 skill total.
Also a full set of t5 teched, marble strength scales will add 230 scalecraft skill. That's a total of 267 skill. Surge of Strength V adds 7 skill. Don't forget potions.
As lvl 95, a full set of scales with self buffs, and an expert dragon lair scaleforge (+90) and you will already be optimal on the expert Focus Wing scale most likely.
For completion sake, these are all the skill levels for the expert dragon scale formulas, in order:
1000 Strength Chest/Head/Foreleg scales
1020 Dexterity Chest/Head/Foreleg scales
1030 Power Chest/Head/Foreleg scales
1050 Focus Chest/Head/Foreleg scales
1060 Armor Chest/Head/Foreleg scales
1080 Health Chest/Head/Foreleg scales
1100 Strength Back/Tail/Hindleg/Wing scales
1120 Dexterity Back/Tail/Hindleg/Wing scales
1130 Power Back/Tail/Hindleg/Wing scales
1150 Focus Back/Tail/Hindleg/Wing scales
1160 Armor Back/Tail/Hindleg/Wing scales
1180 Health Back/Tail/Hindleg/Wing scales
Armor and Health scales are 6 each of Marble and Mithril, so unless you have access to silos near a scaleforge, may not be practical.